bme post

[QofQuimica]

All users may post questions about MCAT, DAT, OAT, or PCAT general chemistry here. We will answer the questions as soon as we reasonably can. If you would like to know what general chemistry topics appear on the MCAT, you should check the MCAT Student Manual (http://www.aamc.org/students/mcat/studentmanual/start.htm)

Acceptable topics:
-general, MCAT-level gen chem.
-particular MCAT-level gen chem problems, whether your own or from study material
-what you need to know about gen chem for the MCAT
-how best to approach to MCAT gen chem passages
-how best to study MCAT gen chem
-how best to tackle the MCAT physical sciences section

Unacceptable topics:
-actual MCAT questions or passages, or close paraphrasings thereof
-anything you know to be beyond the scope of the MCAT

*********

If you really know your gen chem, I can use your help. If you are willing to help answer questions on this thread, please let me know. Here are the current members of the General Chemistry Team:

-QofQuimica (thread moderator): I have my M.S. in organic chemistry and I'm currently finishing my Ph.D., also in organic chemistry. I have several years of university general chemistry TA teaching experience. In addition, I teach general chemistry classes through Kaplan for their MCAT, DAT, OAT, and PCAT courses. On the MCAT, I scored 14 on PS, 43 overall.

-Learfan: Learfan has his Ph.D. in organic chemistry and several years worth of industrial chemistry experience. He scored 13 on the PS section of the MCAT, and 36 overall.

-Sparky Man: Sparky Man has his Ph.D. in physical chemistry. He scored 14 on the PS section of the MCAT, and 36 overall.

-GCT: GCT scored in the 99th percentile on the PCAT. He has also taught introductory physics and general chemistry.

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[QofQuimica]

Well thats certainly the obvious and logical explanation. Unfortunately, you would have made the same mistake as I on kaplan physics section test #4.....

A passage stated that students were given their choice of silver, copper, tin, nickel, and iron to react with HCl to compare their reactivities. The reduction potentials of each metal were given at .8, .34, -.14, -.25, and -.44; respectively.

Obviously tin, copper, and iron turn out to be the most reactive, with their negative reduction potentials, they will be the most easily oxidized to their metal salt CuCl2, etc, and will reduce H+ to H2 gas.

What confused me, is in the explanation of the passage and some of the questions, kaplan states that metals in their ground or elemental state can only be oxidized, not reduced. They will much more readily lose electrons (in this case acting a reducing agent for H+) than gain electrons.

This is confusing because I would expect the metals listed with positive reduction potentials to have the ability to be reduced.

Perhaps QofQuimica could elaborate....

Thanks.

Are you looking at the reduction potentials for M, or for M-? Be careful here; those are two different potentials. Most metals will not want to be reduced further to M-. You would need a very strong reducing agent to do that. Is there a reducing agent that's strong enough? I don't know the answer to that. If there is, my guess is that Kaplan is probably oversimplifying for the sake of teaching you the generally true concept that metals would rather be reducing agents than be reduced themselves.

 

[QofQuimica]

thanks i see what your saying



can i not post questions here??????

Yes, this is where you should post them.

 

[QofQuimica]

3. Electron Affinity. Does this refer to the amount of energy needed to add an electron or the amount released after an electron is added? So in other words if EA is negative does that mean the atom wants the electron or not?

Thanks a lot!

I wrote two posts about periodic trends in the General Chemistry Explanations Thread. Check 'em out.

 

[xanthomondo]

thanks i see what your saying



can i not post questions here??????

posting questions gets you yelled at

maybe someone will answer it before a mod erases it? lol

what question you asking for ill see if i can dig up my AAMC tests

 

[QofQuimica]

posting questions gets you yelled at

maybe someone will answer it before a mod erases it? lol

what question you asking for ill see if i can dig up my AAMC tests

Too late. I'm already here. Please don't post AAMC test questions on SDN, guys. If you want to ask generally about a concept, that's fine. But no exact questions.

 

[xanthomondo]

In question # 59 of the AAMC7,
why is the answer
a saturated solution with 1.3g of undissolved salt instead of supersaturated solution (answer D). I looked up a definition of a supersaturated solution on wiki and in my mind, it describes exactly what the questions stem is asking. maybe the key is that it was left over for several days and the solute adsorbed to a crack or got nucleated?
please help
thank you

A supersaturated solution is obtained when you heat the solution up to increase the solubility and lowering the temperature without it precipitating out...since the question says it is allowed to equilibrate over days, you can assume that the equilibrium of solubility at that temperature takes place...its given to you that 37.7 g/100mL is its solubility, so putting 39 grams in will leave you with 1.3 sitting on the bottom

 

[mcat_study]

mcat_study:

1) Please don't start new threads in the MCAT subforum. Just post your question in the appropriate already existing thread.

2) I don't have the AAMC tests, so I can't answer your question. We are not supposed to post AAMC questions on SDN, but if you can ask about the general concept without posting the specific question, we'll do our best to help.

sorry, i did not know that. i will not do that in the future

 

[osjx-82]

I was wondering if someone could explain the following:


1) an equilibrium process has a ΔS = 0.

2) an equiliburm process has a ΔG = 0 , which means ΔH and TΔS are of equal magnitude.

according to statement 2, ΔS doesn't necessarily need to be zero, as long as TΔS is equal in magnitude to ΔH. Isn't this contradicting statement 1?

 

[mcat_study]

A supersaturated solution is obtained when you heat the solution up to increase the solubility and lowering the temperature without it precipitating out...since the question says it is allowed to equilibrate over days, you can assume that the equilibrium of solubility at that temperature takes place...its given to you that 37.7 g/100mL is its solubility, so putting 39 grams in will leave you with 1.3 sitting on the bottom

thank you

 

[xanthomondo]

I was wondering if someone could explain the following:


1) an equilibrium process has a ΔS = 0.

2) an equiliburm process has a ΔG = 0 , which means ΔH and TΔS are of equal magnitude.

according to statement 2, ΔS doesn't necessarily need to be zero, as long as TΔS is equal in magnitude to ΔH. Isn't this contradicting statement 1?

All of those statements are true...its because ΔH =0 at equilibrium also

 

[pezzang]

What is the difference between network covalent crystal and molecular crystal? They are both made of nonpolar atoms. But I am not sure if molecular, despite having all nonpolar atoms, has covalent bonds. How would you tell if the bond is covalent or not among bonds consisting of only nonpolar atoms? Thanks!

 

[pezzang]

why is "the strongest bonds present in a glass of water" and "the strongest bonds MADE by water (molecules) in a glass of water" different? Why is that even though water molecules CONSIST of covalent H-O bonds, they do not MAKE covalent bonds (with other molecules) but they make hydrogen bonds? Thanks.

 

[t2oo5]

why is "the strongest bonds present in a glass of water" and "the strongest bonds MADE by water (molecules) in a glass of water" different? Why is that even though water molecules CONSIST of covalent H-O bonds, they do not MAKE covalent bonds (with other molecules) but they make hydrogen bonds? Thanks.

Correct me if I'm wrong...but I believe the strongest bonds in a glass of water are the H-O-H Bonds to the O-Si-O (silicon dioxide) bonds in glass. The dipole-dipole attractions are greater from SiO2-H2O as compared to the attraction of H2O-H2O. The dipole-dipole attractions are greater with the partial negative of the oxygens in SiO2 and the Partial Positive of hydrogens in H2O, than the Hydrogen bonding that exists among water molecules.

And to answer the second part, as to why H2O molecules don't make covalent bonds with anything else, well think about it: the octet of O is filled due to the two covalent bonds with the hydrogens, and hydrogens S shell is filled as well, so theres really no room for any bonds. Hydrogen bonds happen because the partial positive (of Hydrogens) and partial negatives (of oxygen) are attracted to the opposing charges of each molecule. So the Oxygen of a water molecule is attracted to a hydrogen of another water molecule. And the hydrogens of one water molecule are each attracted to two different oxygens of other water molecules.
Hope this answers your question.

 

[H and D]

How do you determine the relative solubilities of different anions?
For instance how would I know that CO3-2 will precipitate out of solution before I- will?
I know the general guidlines for determining if a compound will be soluble but I am not finiding any information in my EK book on comparing solubilities of various ions.
Thanks for the help (it is much appreciated).

 

[pezzang]

How do you determine the relative solubilities of different anions?
For instance how would I know that CO3-2 will precipitate out of solution before I- will?
I know the general guidlines for determining if a compound will be soluble but I am not finiding any information in my EK book on comparing solubilities of various ions.
Thanks for the help (it is much appreciated).

In general, any compounds containing CO3 2- are insoluble while compounds with I- (or other halogens) are mostly soluble unless bound to Ag+, Hg2+ or Pb2+. From this background knowledge, we can conclude that CO3 2- will precipiate out regardless of its related concentration while the precipitation of I- will depend on its concentration in respect to its solubility. Hence, CO3 2- will precipiate out the first and I- next according to the concentration. Hope it helps!

 

[pezzang]

Which one has the highest boiling point?

a) SiH4
b) H2Te
c) H2S
d) SnH4

I realize we need to compare intermolecular forces and MW when comparing boiling point. I chose c) because H-S has the highest polarity. However, the answer said b), basically giving more emphasis on MW than polarity. So the question is...if no hydrogen bond (the strongest intermolecular force) is absent, is it safe to assume that MW overpowers intermolecular forces? Thanks for your help!

 

[QofQuimica]

Which one has the highest boiling point?

a) SiH4
b) H2Te
c) H2S
d) SnH4

I realize we need to compare intermolecular forces and MW when comparing boiling point. I chose c) because H-S has the highest polarity. However, the answer said b), basically giving more emphasis on MW than polarity. So the question is...if no hydrogen bond (the strongest intermolecular force) is absent, is it safe to assume that MW overpowers intermolecular forces? Thanks for your help!

Probably, if the difference in MW is great. Be careful, though, because the polarity of the individual molecules in dipole-dipole interactions is also important. I'm not sure that you can categorically make that assumption in every case for that reason.

 

[pezzang]

QofQuimica, can you answer this too?
What is the difference between network covalent crystal and molecular crystal? They are both made of nonpolar atoms. But I am not sure if molecular, despite having all nonpolar atoms, has covalent bonds. How would you tell if the bond is covalent or not among bonds consisting of only nonpolar atoms? Thanks!

 

[QofQuimica]

QofQuimica, can you answer this too?
What is the difference between network covalent crystal and molecular crystal? They are both made of nonpolar atoms. But I am not sure if molecular, despite having all nonpolar atoms, has covalent bonds. How would you tell if the bond is covalent or not among bonds consisting of only nonpolar atoms? Thanks!

I don't know; I've never heard of molecular crystal. Network covalent is where you have an extended molecule of nonmetallic atoms. Think of something like a diamond, where the carbons are all bonded to one another in a tetrahedral orientation that extends in every direction and includes millions of atoms.

 

[Cloudcube]

Hi Everyone! This really is an awesome thread and a great help.

I was wondering if anyone would be able to help me with aromaticity? Could someone please take the time to explain, in detail, how aromaticity works. I feel like I'm missing a lot of stuff. Mainly, I've been having trouble grasping how the hybrid orbitals fit in with aromaticity and how I use those to tell me whether something is aromatic. For something that has groups attached to it (like N or O) I find it much more difficult to tell whether it's aromatic. I realize this is probably a simple question, but I'd really appreciate it if someone took the time to answer.

Thanks!

 

[googlinggoogler]

Hi, I am kinda confused about the solubility rules. Silver is insoluable right? So if I have a solution of AgCl (s) in equilibrium, and I add NaCl, how do I form more AgCl? The AgCl could never have broken up into Ag+ and Cl- in the first place right? I'm sorry, maybe I'm just stupid. But I don't understand how I could even have a solution of AgCl in the first place.

 

[pezzang]

Hi, I am kinda confused about the solubility rules. Silver is insoluable right? So if I have a solution of AgCl (s) in equilibrium, and I add NaCl, how do I form more AgCl? The AgCl could never have broken up into Ag+ and Cl- in the first place right? I'm sorry, maybe I'm just stupid. But I don't understand how I could even have a solution of AgCl in the first place.

Solubility is a relative concept. Insolubility doesn't mean that the compound such as AgCl won't dissolve in a solution at all. It will dissolve but the degree of solubiity is so low (I believe the cutoff line is 0.001) that we call it insoluble compound. Hence, when NaCl is added to a solution containing AgCl, Na+ and Cl- will dissolve in a solution but as Cl- reacts with Ag+, the precipitate will form as the concentrations of Ag+/Cl- goes above their solubility. Anybody can correct me or add if necessary. Good luck!

 

[googlinggoogler]

Thanks for the explanation! That makes a lot more sense Good luck to you too

 

[QofQuimica]

Hi Everyone! This really is an awesome thread and a great help.

I was wondering if anyone would be able to help me with aromaticity? Could someone please take the time to explain, in detail, how aromaticity works. I feel like I'm missing a lot of stuff. Mainly, I've been having trouble grasping how the hybrid orbitals fit in with aromaticity and how I use those to tell me whether something is aromatic. For something that has groups attached to it (like N or O) I find it much more difficult to tell whether it's aromatic. I realize this is probably a simple question, but I'd really appreciate it if someone took the time to answer.

Thanks!

I posted for you in the organic thread, and also added aromaticity to the organic explanations thread. You don't need to post your question in more than one thread; I read all of them.

 

[pezzang]

I took Kaplan diagnostic last yr and i was just reviewing it and came across this question:

Zn(s) + NiCl3(aq) -> ZnCl2(aq) + Ni(s)

Which material serves as the anode in the electrochemical reaction?
a. ZnCl2
b. NiCl3
c. Hg(l)
d. Zn(s)

The answer is evidently d. right? because oxidation takes place in anode and Zn is oxidized from 0 to +2. And by the material, the answer should be the material in the reactant side. The answer key said b, and I have no idea why? Am I too burned out to solve this easy problem or is it a typo?

 

[pezzang]

Sorry, another question,
What is the mass of 1L of air at STP given that air consists of 78% nitrogen, 21% oxygen and 1% argon by volume?
The answer of 14.69g and i have no idea where to start except that STP is for 22.4L, 298K, 1atm and changing from 22.4L to 1L gives the fractional change of n(# moles) but I don't know the MW of air...

Also, why is it that at 270K, 1 mole of O2 has higher KE than 1 mole of He while 1 mole of N2 doesn't have more KE than 1 mole of O2? I thought T is the measure of average KE. So if T is the same for both, shouldn't KE(avg) be the same? (that's why for larger mass, v is smaller as proven by graham's law)

Gen Chemistry just become my hardest subject in MCAT....

 

[googlinggoogler]

I took Kaplan diagnostic last yr and i was just reviewing it and came across this question:

Zn(s) + NiCl3(aq) -> ZnCl2(aq) + Ni(s)

Which material serves as the anode in the electrochemical reaction?
a. ZnCl2
b. NiCl3
c. Hg(l)
d. Zn(s)

The answer is evidently d. right? because oxidation takes place in anode and Zn is oxidized from 0 to +2. And by the material, the answer should be the material in the reactant side. The answer key said b, and I have no idea why? Am I too burned out to solve this easy problem or is it a typo?

I agree, that looks like a typo to me too.

 

[QofQuimica]

I agree, that looks like a typo to me too.

I agree too. Ni has been reduced, so it's the cathode, not the anode.

 

[QofQuimica]

Sorry, another question,
What is the mass of 1L of air at STP given that air consists of 78% nitrogen, 21% oxygen and 1% argon by volume?
The answer of 14.69g and i have no idea where to start except that STP is for 22.4L, 298K, 1atm and changing from 22.4L to 1L gives the fractional change of n(# moles) but I don't know the MW of air...

Also, why is it that at 270K, 1 mole of O2 has higher KE than 1 mole of He while 1 mole of N2 doesn't have more KE than 1 mole of O2? I thought T is the measure of average KE. So if T is the same for both, shouldn't KE(avg) be the same? (that's why for larger mass, v is smaller as proven by graham's law)

Gen Chemistry just become my hardest subject in MCAT....

Hmm, I would say that the first question is a sort of Dalton's Law problem. In other words, you need to act as though you had 1 L of each gas alone, figure out what the mass of it would be, and then multiply by the proportion of that gas. (That is, figure out how much a liter of nitrogen gas would weigh, then multiply by 0.78. Repeat for the other gases using the corresponding proportions.)

Second question: If you're assuming that they're ideal, two gases at the same temperature should have the same KE. (KE = 3/2 RT) Maybe they're assuming non-ideality?

 

[Dr Durden]

Sorry, another question,
What is the mass of 1L of air at STP given that air consists of 78% nitrogen, 21% oxygen and 1% argon by volume?
The answer of 14.69g and i have no idea where to start except that STP is for 22.4L, 298K, 1atm and changing from 22.4L to 1L gives the fractional change of n(# moles) but I don't know the MW of air...

The molecular weight of air is about 29 g/mol. It's simply weighting each individual gas's molecular weight like 0.78*28 + 0.21*32 + 0.01*40 = 29. As you know, at STP 22.4 L = 1 mole of gas. Thus (29 g/mol)(1mol/22.4L) = 1.29 g/L, the standard density. Since you have 1 L, you should get 1.29 g. I don't know where 14.69 g came from. That mass looks closer to half of the STP volume.

Edit: I see Q beat me to it. I'll leave my post up just so we can check our work.

 

[pezzang]

Hmm, I would say that the first question is a sort of Dalton's Law problem. In other words, you need to act as though you had 1 L of each gas alone, figure out what the mass of it would be, and then multiply by the proportion of that gas. (That is, figure out how much a liter of nitrogen gas would weigh, then multiply by 0.78. Repeat for the other gases using the corresponding proportions.)

Second question: If you're assuming that they're ideal, two gases at the same temperature should have the same KE. (KE = 3/2 RT) Maybe they're assuming non-ideality?

The molecular weight of air is about 29 g/mol. It's simply weighting each individual gas's molecular weight like 0.78*28 + 0.21*32 + 0.01*40 = 29. As you know, at STP 22.4 L = 1 mole of gas. Thus (29 g/mol)(1mol/22.4L) = 1.29 g/L, the standard density. Since you have 1 L, you should get 1.29 g. I don't know where 14.69 g came from. That mass looks closer to half of the STP volume.


I definitely agree with what Dr.Durden did regarding the first problem. I re-did the problem and it seems the answer is 1.3g. One of the answer choice was 1.3g but the actual answer was 14.8g. Is this again a typo? If it is, it seems that people don't do well on Kaplan diagnostic test (MM5012J) because wrong answers are correct.... I have another five/six problems to ask but I am pretty sure they are typos... Thank you very much, QofQuimica and Dr.Durden.....

 

[QofQuimica]

The molecular weight of air is about 29 g/mol. It's simply weighting each individual gas's molecular weight like 0.78*28 + 0.21*32 + 0.01*40 = 29. As you know, at STP 22.4 L = 1 mole of gas. Thus (29 g/mol)(1mol/22.4L) = 1.29 g/L, the standard density. Since you have 1 L, you should get 1.29 g. I don't know where 14.69 g came from. That mass looks closer to half of the STP volume.


I definitely agree with what Dr.Durden did regarding the first problem. I re-did the problem and it seems the answer is 1.3g. One of the answer choice was 1.3g but the actual answer was 14.8g. Is this again a typo? If it is, it seems that people don't do well on Kaplan diagnostic test (MM5012J) because wrong answers are correct.... I have another five/six problems to ask but I am pretty sure they are typos... Thank you very much, QofQuimica and Dr.Durden.....

Might be. Try checking with Kaplan; they have a list of the errata.

 

[pezzang]

I was re-reading about intra/inter molecular forces from your post, QofQuimica today cuz I forgot them all. After the reading, I still wasn't sure which occurs in larger scale, intra or inter? I initially thought it was intra that was smaller (WITHIN A molecule) and inter was bigger (Among moleculeS). But the reading made me think otherwise. Can you confirm on this please? Thanks!

 

[QofQuimica]

I was re-reading about intra/inter molecular forces from your post, QofQuimica today cuz I forgot them all. After the reading, I still wasn't sure which occurs in larger scale, intra or inter? I initially thought it was intra that was smaller (WITHIN A molecule) and inter was bigger (Among moleculeS). But the reading made me think otherwise. Can you confirm on this please? Thanks!

I'm not sure that I understand your question. What do you mean by "occurs in a larger scale?" If you mean which are stronger, then INTRAmolecular interactions (bonds) are stronger than INTERmolecular interactions. If you mean in terms of distance, then bonds are shorter than intermolecular interactions. If I haven't hit what you're asking, can you clarify?

 

[deleted106503]

I'm not sure that I understand your question. What do you mean by "occurs in a larger scale?" If you mean which are stronger, then INTRAmolecular interactions (bonds) are stronger than INTERmolecular interactions. If you mean in terms of distance, then bonds are shorter than intermolecular interactions. If I haven't hit what you're asking, can you clarify?

AFAIK, inter-molecular forces are forces between molecules. The force between 2 molecules given by the equation F=kq1q2/r^2. If two molecules are in an isolated system by themselves, you could also find the gravitational force between them.

Intra-molecular forces are the forces within a molecule like the force an electron feels against a proton and shielding effects, vibrational energy, translational energy, rest mass energy, etc, all better known as "U." If this isn't the case, I have no idea...

Which one occurs on a bigger scale??? Do they mean which force is the largest?

 

[QofQuimica]

AFAIK, inter-molecular forces are forces between molecules. The force between 2 molecules given by the equation F=kq1q2/r^2. If two molecules are in an isolated system by themselves, you could also find the gravitational force between them.

Intra-molecular forces are the forces within a molecule like the force an electron feels against a proton and shielding effects, vibrational energy, translational energy, rest mass energy, etc, all better known as "U." If this isn't the case, I have no idea...

Which one occurs on a bigger scale??? Do they mean which force is the largest?

I am still not sure what s/he's trying to ask; pezzang will have to tell us whether we've answered the question.

 

[pezzang]

I am still not sure what s/he's trying to ask; pezzang will have to tell us whether we've answered the question.

I meant to ask which force covers more space but now I realize intermolecular forces cover less area because it covers the space outside of molecules, which is much smaller than the molecules themselves. Am I correct in this regard? And QofQuimica, can you elaborate on how the distance between molecules is smaller than the molecules themselves? I have trouble distinguishing the relative size of the moelcules and the space between them. Thanks again!

 

[pezzang]

AFAIK, inter-molecular forces are forces between molecules. The force between 2 molecules given by the equation F=kq1q2/r^2. If two molecules are in an isolated system by themselves, you could also find the gravitational force between them.

Intra-molecular forces are the forces within a molecule like the force an electron feels against a proton and shielding effects, vibrational energy, translational energy, rest mass energy, etc, all better known as "U." If this isn't the case, I have no idea...

Which one occurs on a bigger scale??? Do they mean which force is the largest?

Thanks WilliamsF1! Can you just say a few more words about how to find the gravitational force between molecules in an isolated system? I want to be sure what you are talking about. Thanks!

 

[QofQuimica]

I meant to ask which force covers more space but now I realize intermolecular forces cover less area because it covers the space outside of molecules, which is much smaller than the molecules themselves. Am I correct in this regard? And QofQuimica, can you elaborate on how the distance between molecules is smaller than the molecules themselves? I have trouble distinguishing the relative size of the moelcules and the space between them. Thanks again!

Hmm, I think you've got them reversed. The size of the molecules will be smaller than the size of the space that contains them. Or, at most, they will be the same size.

How much smaller the molecules are than the space around them is going to depend on the phase of matter you have and on the type of molecule, but it is probably safe to say for most cases that you'd be interested in for MCAT purposes that the intermolecular distances will be much larger than the molecules themselves. In general, gases will have huge distances between molecules, liquids will have smaller distances, and solids will have smaller distances yet. Also, nonpolar molecules will tend to have larger distances between them compared to polar molecules due to having smaller intermolecular forces. This is assuming that the MW is similar, of course. And then you also need to consider the issue of branching if you have to isomeric hydrocarbons. Clear as mud?

 

[WestTexasRambler]

Quick exposure of my poor chemistry acumen.....

When adding equivalent molar concentration of any acid or base, say 10 mL of .1M HCl and 10 mL of .1M NH3, is there a complete neutralization? Do they have to be both strong acids to be a complete neutralization ie pH of 7 or is my definition of complete neutralization wrong?


Thank you,


'Rambler

 

[deleted106503]

HCl is a strong acid and NH3 is a weak base. Since it's in eqi molar concentrations, when mixed, it will be a weak acid. At least that's what EK taught me for salts being dissolved in water. I would like to know: how do I find the pH of that solution you listed?

 

[WestTexasRambler]

HCl is a strong acid and NH3 is a weak base. Since it's in eqi molar concentrations, when mixed, it will be a weak acid. At least that's what EK taught me for salts being dissolved in water. I would like to know: how do I find the pH of that solution you listed?

Considering your answer: So is the resulting solution considered to be completely neutralized or is it that complete neutralization results in a solution with a pH of 7.

Your Q:
Not sure but I would expect that they would have to give you the the resulting [NH4Cl] as well as some Ka value for ammonium but I am not very good at Gchem so best wait for someone with authority.

G'luck.

 

[deleted106503]

1 M of NaH2PO4 is in solution. The pKa of NaH2PO4 is given as 7.21 and pKb is 11.84 (EK got the pKb from the 14 - pKa of H3PO4). Then they said that these two numbers show that NaH2PO4 is a better acid than base. This part I don't understand. The pKb is high and the pKa is high, too. Why isn't it basic (instead of pH 3.6)?

 

[pezzang]

I am reading EK chem and I have trouble understanding this in chpter 1 under the heading "1-17 Energy levels of Electrons" on pg 18. It says "Like the energy between bonding atoms, the energy between the electron and the nucleus increaes from a negative to zero as the electron moves to an infinite distance away from the nucleus."
I have been having trouble understanding "electrostatic potential energy" created due to the bonding between electrons and nucleus. I have trouble envision it even though I know what potential energy is in physics. Can you please explain how this electrostatic potential energy is created and how its signs and magnitude change in respect to the bond length? Also, I hope to understand the quote above; especially, how the energy changes from negative to zero as the electron moves to an infinite distance away from the nucleus (thus increasing). Thanks, QofQuimica and anybody else who can answer.

 

[Creightonite]

Hi, another question. I was doing EK's 1001 problems but got the relationship between rate of the reaction and catalyst. The anwer sheet tells me that the catalyst changes the rate of the reaction by changing the rate constant. I thought that the only thing that catalyst does is to lower the activation energy. Can someone clear this up?

 

[deleted106503]

Hi, another question. I was doing EK's 1001 problems but got the relationship between rate of the reaction and catalyst. The anwer sheet tells me that the catalyst changes the rate of the reaction by changing the rate constant. I thought that the only thing that catalyst does is to lower the activation energy. Can someone clear this up?

I was a bit confused by this as well. From what I gathered in the EK Inorgo Book is that a catalyst changes the rate constant indirectly by changing the energy of activiation. Here's the formula for the rate constant: k=zpe^(-Ea/RT) where Ea is activation energy. So I'm assuming the catalyst does it indirectly by lowering Ea and not directly chaning the rate constant. I find this funny because on page 30 of EK Chem it says "The value of the rate constant depends upon pressure, catalysts, and temperature." Don't worry, I missed a few of EK questions becaues of confusion from the lack of details.

 

[Creightonite]

Just from looking at that formula, if E is lowered that causes the rate constant to increase. Hm... I think this goes against what I learned in Gen Chem classes.

 

[DesiMcatAcer]

Hi, another question. I was doing EK's 1001 problems but got the relationship between rate of the reaction and catalyst. The anwer sheet tells me that the catalyst changes the rate of the reaction by changing the rate constant. I thought that the only thing that catalyst does is to lower the activation energy. Can someone clear this up?

Yes, it's true that the catalyst will lower Ea, which inturn may effect (b/c in the equation below A is also function of Ea) the rate constant, because the rate constant is a function of Ea. Since, k=Aexp(-Ea/RT), is Arrhenius form of equation, but in many situations it is also a function of Pressure and other factors because there is other forms of this equation too.
If you want to answer in short, catalyst lowers Ea, if Ea is lowered then according to k=Aexp(-Ea/RT), then the value of k will become higher if you do the math, and since rate is directly proportional to k, then it increases! Hope it helps.
-Desi

 

[brownie786]

So... I came across this question in the PR workbook...
" Given that all of the following electrodes have a mass of 1 gram, which of these electrodes should be used to construct a high current battery?"
A.) Solid cylinder electrode
B.) Thick plate electrode
C.) Spherical electrode
D.) Foil electrode

The passage tells you that the electrode with the greatest surface area gives you the highest current.
The answer is D but there isn't really any sort of an explanation... but a foil electrode isn't ever mentioned in the passage nor have I come across it anywhere... is this something that I'm supposed to just know??

Oh and for anyone who wants to look it up: passage 84 #3

 

[deleted106503]

So... I came across this question in the PR workbook...
" Given that all of the following electrodes have a mass of 1 gram, which of these electrodes should be used to construct a high current battery?"
A.) Solid cylinder electrode
B.) Thick plate electrode
C.) Spherical electrode
D.) Foil electrode

The passage tells you that the electrode with the greatest surface area gives you the highest current.
The answer is D but there isn't really any sort of an explanation... but a foil electrode isn't ever mentioned in the passage nor have I come across it anywhere... is this something that I'm supposed to just know??

Oh and for anyone who wants to look it up: passage 84 #3

First off let me say that I'm using EK and this material is not in our books.

Second, foil makes the most sense. If you have 1 gram of substance, how will you make it have the most surface area? Most likely flatten it out as much as you can, hopefully to 1 e- thick, but that's not really possible. The only one that is as flat as can be with the 1g mass requirement is the foil.