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]

I am having a lot of trouble with calorimetry passages. The concept of bomb/coffee cup calorimeters doesn't seem so bad but I have a lot of trouble applying it to questions.

I will add calorimetry to the list of topics for the gen. chem. explanations thread. Basically, you need to remember that the heat lost by one substance is equal to the heat gained by another. Heat is a form of energy, so it should never be created nor destroyed.

 

[QofQuimica]

Hi, after going over the redox chapter, i basically understood everything, but I'm having problems with passages that deal with the application of redox reactions in real life. Say, if metal corrodes, how do you know if the metal is being reduced or oxidized? There was another passage where they did an experiment with metals in acids without any reduction potentials table, in this case, how do you know that they will react and that it's a redox reaction? and if so, that which one is reduced, and which one is oxidized (without any reduction potentials given)?

During corrosion, the metals get oxidized and the oxygen gets reduced. The best place to start when considering any problem like this is to write down a reaction for what is occurring. This will help you reason out each partner's role in the redox reaction. Consider this generic reaction that is occurring for an oxidation of a metal:

M (s) + O2 (g) -> M^xO^y

where x and y are positive integers. Your metal starts out in its standard state (solid metal), which has an oxidation number = 0. The same is true for the oxygen gas. After reaction to form the metal oxide, your metal is now a cation and your oxygen now has a formal charge of -2. Thus, the metal has been oxidized, and the oxygen has been reduced.

In the case of metals being dissolved in acid, the acid ends up forming H2 gas. Try to write an equation like we did before, and you should find that again the metal gets oxidized while the H gets reduced:

M + H+ -> H2 + M^n+

where n is some positive integer.

 

[freesolo]

im not sure if i can post this its examkrackers question of the day, just wondering why the answer isnt 2000 ml but 1900 ml

100 mL of 1 molar HCl is to be diluted to 0.05 molar HCl. How much water should be added?

im using the M1V1=M2V2 this damn formula never works it seems =p.

 

[Nutmeg]

im not sure if i can post this its examkrackers question of the day, just wondering why the answer isnt 2000 ml but 1900 ml

100 mL of 1 molar HCl is to be diluted to 0.05 molar HCl. How much water should be added?

im using the M1V1=M2V2 this damn formula never works it seems =p.

V1 and V2 in that equation are the total volumes. You end up with V2 = 2000 ml, but you then must realize that they are asking for how much water to add to the 100 ml. Hence, you would add 2000 - 100 = 1900 ml.

 

[faluri]

Why does increasing the vapor pressure DECREASE the boiling point? I thought a liquid boiled when atmospheric pressure and the vapor pressure of the liquid were equal. So if the vapor pressure is somehow increased, how does that translate to a shift in the boiling point???

 

[QofQuimica]

Why does increasing the vapor pressure DECREASE the boiling point? I thought a liquid boiled when atmospheric pressure and the vapor pressure of the liquid were equal. So if the vapor pressure is somehow increased, how does that translate to a shift in the boiling point???

I don't understand your question. If you raise the vapor pressure above the ambient or atmospheric pressure, then boiling will occur. But changing the vapor pressure doesn't change the boiling point, which is constant for a substance at a given temperature and pressure. Changing the atmospheric pressure does affect the boiling point, but not the way you described. Nutmeg's post above explains this relationship. You can also think of it this way: if you lower the atmospheric pressure, you have lowered the vapor pressure that you need to reach for boiling to occur. If you put liquid water into a vacuum, it will vaporize instantly. Why? Because there is no atmospheric pressure, so the vapor pressure of the water doesn't have to be very high for boiling to occur.

 

[faluri]

maybe phrased a bit differently,

if you lets say add NaCl to water, the boiling point increases to greater than 100 degrees celsius. Why does this then mean that the atmospheric pressure must be less than 1 atm?

 

[QofQuimica]

maybe phrased a bit differently,

if you lets say add NaCl to water, the boiling point increases to greater than 100 degrees celsius. Why does this then mean that the atmospheric pressure must be less than 1 atm?

The atmospheric pressure has not changed! It is constant for a given set of conditions. What you've done by adding salt to the water is to lower the VAPOR PRESSURE of that water. Since you need to have your vapor pressure equal to the atmospheric pressure before boiling will occur, then you will need to boil your water at a higher temperature in order to raise the vapor pressure up to the level of the atmospheric pressure.

 

[MDhopeful023]

QofQuimica,

This probably is a stupid question but I was wondering if there are atoms that can have more than four bonds... greater than an octet??? Do we have to know these atoms that can have more than an octet??

I was just curious....

 

[QofQuimica]

QofQuimica,

This probably is a stupid question but I was wondering if there are atoms that can have more than four bonds... greater than an octet??? Do we have to know these atoms that can have more than an octet??

I was just curious....

It's not a stupid question at all, and yes, there are. Elements in the third row of the periodic table and below can exceed their octets by hybridizing with their d-orbitals. For example, phosphorus can hybridize to form 5 sp3d orbitals, and have five bonds; this is called trigonal bipyramidal geometry. Sulfur can form 6 sp3d2 orbitals, have six bonds, and take on octahedral geometry. Note that second row elements can NEVER exceed their octets. So don't ever make pentavalent carbons, nitrogens, or oxygens!

 

[MDhopeful023]

It's not a stupid question at all, and yes, there are. Elements in the third row of the periodic table and below can exceed their octets by hybridizing with their d-orbitals. For example, phosphorus can hybridize to form 5 sp3d orbitals, and have five bonds; this is called trigonal bipyramidal geometry. Sulfur can form 6 sp3d2 orbitals, have six bonds, and take on octahedral geometry. Note that second row elements can NEVER exceed their octets. So don't ever make pentavalent carbons, nitrogens, or oxygens!

I just looked at the periodic table and I'm a little confused.. you say d orbitals for P and S but they are in their p orbital... I thought that they can fill up their p subshell by sharing electrons but how do they have a d subshell???

Thanks for clarifying this ... I got a question wrong do to this (it was a VSPER one)

 

[QofQuimica]

I just looked at the periodic table and I'm a little confused.. you say d orbitals for P and S but they are in their p orbital... I thought that they can fill up their p subshell by sharing electrons but how do they have a d subshell???

They use their empty 3d orbitals. This is why the third row elements can do it, but the second row elements cannot (there are no 2d orbitals, right?)

 

[Scrub MD]

For solubility guidelines (i.e. N03 soluble with NH4 and alkali metals). Did you suck it up and memorize them or only a few or have a good mnemonic?

 

[QofQuimica]

For solubility guidelines (i.e. N03 soluble with NH4 and alkali metals). Did you suck it up and memorize them or only a few or have a good mnemonic?

Solubility mnemonic

 

[TheGuy2000]

Another question about delta H values from Kap's FL 3

Calculate the delta H of formation of Fe2O3

6Fe2O3 --> 4 Fe3O4 + O2 +472 kJ/mol
3Fe + 2O2 ---> Fe3O4 -1118.4 kj/mol

Choices are in kj/mol

667
-1559
-824
-667

The answer is C, but I had no clue how they got it.

 

[QofQuimica]

Another question about delta H values from Kap's FL 3

Calculate the delta H of formation of Fe2O3

6Fe2O3 --> 4 Fe3O4 + O2 +472 kJ/mol
3Fe + 2O2 ---> Fe3O4 -1118.4 kj/mol

Choices are in kj/mol

667
-1559
-824
-667

The answer is C, but I had no clue how they got it.

This is a Hess's Law problem. Hess's Law tells you that since enthalpy is a state function, we don't care about the path taken to go from starting materials (Fe metal and O2 gas) to product (Fe2O3). Start by writing out the net equation of the reaction for formation of Fe2O3 from its constituent elements. You then need to manipulate the step equations (given in the problem) so that the intermediate (Fe3O4) will cancel out and give you the net equation. Don't forget to multiply the deltaH value by the same amount as its equation. Also, you will have to turn one equation backwards (multiply by negative one). Give it a shot yourself, and let me know if you need more help.

 

[Will Hunting]

This is a Hess's Law problem. Hess's Law tells you that since enthalpy is a state function, we don't care about the path taken to go from starting materials (Fe metal and O2 gas) to product (Fe2O3). Start by writing out the net equation of the reaction for formation of Fe2O3 from its constituent elements. You then need to manipulate the step equations (given in the problem) so that the intermediate (Fe3O4) will cancel out and give you the net equation. Don't forget to multiply the deltaH value by the same amount as its equation. Also, you will have to turn one equation backwards (multiply by negative one). Give it a shot yourself, and let me know if you need more help.

Q, your knowledge of the sciences is amazing. You knock down every question. Keep up the good work.

 

[hippocampus]

for anode and cathode, why is there a diff between the physics anode and cathode, and the chemistry anode and cathode? they always change the signs .. why?

 

[QofQuimica]

for anode and cathode, why is there a diff between the physics anode and cathode, and the chemistry anode and cathode? they always change the signs .. why?

Because physicists solve these problems from the perspective of current flow, which is positive, and chemists solve them from the perspective of electron flow, which is negative. I don't know why the conventions are different, but I've always thought that the chemistry one makes more sense, since current flows in the direction of imaginary positive charges, as opposed to ostensibly real electrons. But I may be somewhat biased.

 

[stoleyerscrubz]

I finished studying all of EK and I have the basics down for the most part but there are a number of things mentioned in the AAMC MCAT Topic List that were not covered in EK materials. Could you comment on what we need to know about some of these. Thanks.

bonding-the ionic bond(electrostatic forces between ions)
E=lattice energy
and
Force attraction=R(n+e)(n-e)/(dxd)

 

[Twitch]

How many moles of NaOH must be added to 1.0L of 2.0M acetic acid to produce a buffered solution at pH=pKa?

The answer is 1, but I'm having a hard time buying the stmt:
[C2H3O2-] + [HC2H3O2] = 2.0M

 

[QofQuimica]

I finished studying all of EK and I have the basics down for the most part but there are a number of things mentioned in the AAMC MCAT Topic List that were not covered in EK materials. Could you comment on what we need to know about some of these. Thanks.

bonding-the ionic bond(electrostatic forces between ions)
E=lattice energy
and
Force attraction=R(n+e)(n-e)/(dxd)

I would say you don't need to know about any of these things in great detail, and you probably already know most of this anyway.

Lattice energy refers to the energy of attraction between the cations and anions of a salt; the greater this attaction is, the higher the lattice energy will be. You can also think of it as the energy required to pull the ions of a salt apart from one another. Smaller, highly charged ions tend to form salts with greater lattice energies. Salts with higher lattice energies tend to be less soluble because the ions don't want to separate. You can use Coulomb's Law (same as from physics) to calculate the ionic force between cations and anions if you know the van der Waals radii of the ions and the charges on the ions.

 

[QofQuimica]

How many moles of NaOH must be added to 1.0L of 2.0M acetic acid to produce a buffered solution at pH=pKa?

The answer is 1, but I'm having a hard time buying the stmt:
[C2H3O2-] + [HC2H3O2] = 2.0M

You start out with 1 L x 2 mol/L = 2 moles of acid (HA). So to reach the pKa, you have to add one mole of NaOH, leaving you with one mole of conjugate acid (HA) and one mole of conjugate base (A-). Since you have one liter of solution, then you end up with 1 mol/L of HA and 1 mol/L of A-. Add the two individual molarities to get the total molarity of particles....though I'm a little confused myself about what the point was in doing that. Were they trying to prove to you that the total molarity stays constant?

 

[hippocampus]

Because physicists solve these problems from the perspective of current flow, which is positive, and chemists solve them from the perspective of electron flow, which is negative. I don't know why the conventions are different, but I've always thought that the chemistry one makes more sense, since current flows in the direction of imaginary positive charges, as opposed to ostensibly real electrons. But I may be somewhat biased.

thanks for the response.

so does that mean chemist make the anode plus and the cathode minus, while physicist make it the opposite? so does that mean chemist make it more like an electrolytic cell? im confused... can you give me some examples?

 

[Twitch]

Hi Q - you're the best! Thanks for taking for everything you do for us. I guess the part I was having difficulty was with the statement that the sum of the concentration of acetic acid + it's conjugate = 2.0 M which is the stated conc. of acetic acid (just by itself)

I guess one way to look at it is that acetic acid could exist in an aq soln as it's conjugate [A-] & [HA] so total conc is the sum of the two? The other reason I was having a hard time with this is because when I use the H/H eqn:
pH = pKa + log(base/acid)
since pKa = pH, thus
base/acid = 1

Now my point is this can mean that moles of base = moles of acid (apparently a true statement). But it could also mean [base]/[acid] = 1 in which case [A-] = [HA] and so how can [HA] + [A-] = 2??

Help!

You start out with 1 L x 2 mol/L = 2 moles of acid (HA). So to reach the pKa, you have to add one mole of NaOH, leaving you with one mole of conjugate acid (HA) and one mole of conjugate base (A-). Since you have one liter of solution, then you end up with 1 mol/L of HA and 1 mol/L of A-. Add the two individual molarities to get the total molarity of particles....though I'm a little confused myself about what the point was in doing that. Were they trying to prove to you that the total molarity stays constant?

 

[QofQuimica]

thanks for the response.

so does that mean chemist make the anode plus and the cathode minus, while physicist make it the opposite? so does that mean chemist make it more like an electrolytic cell? im confused... can you give me some examples?

I meant that the sign of the flow of charge, and therefore its direction, is reversed. For a galvanic (spontaneous) cell, a chemist would say that the electrons move from the negative anode to the positive cathode (reduction always occurs at the cathode, so the electrons always go there). A physicist would say that the current flows from the positive cathode to the negative anode. So the electrode signs should be the same, but the flow of charges is reversed, since the signs of the charges of the electrons and the current are opposite. If the electrode signs change, that means you have an electrolytic (nonspontaneous) cell; those cells have a positive anode and a negative cathode.

 

[QofQuimica]

The other reason I was having a hard time with this is because when I use the H/H eqn:
pH = pKa + log(base/acid)
since pKa = pH, thus
base/acid = 1

Now my point is this can mean that moles of base = moles of acid (apparently a true statement). But it could also mean [base]/[acid] = 1 in which case [A-] = [HA] and so how can [HA] + [A-] = 2??

In this case, [A-] = [HA] = 1 M. So when you divide them, you'll find that (1/1) = (base/acid) gives a ratio of one, but when you add [acid] + [base], you'll get (1 + 1 = 2). However, if the concentrations were anything else (i.e., not equal to one), then you wouldn't be able to add them together to get two like you can here, but you'd still get one when you divided them. So, say you ended up with 1.5 M HA and 1.5 M A-. You'd still get (A-/HA) = (1.5/1.5) = 1, but now they'd add up to 3 M total concentration.

 

[thadarknyte]

we need to know about logarithms and how to compute them. I have very forgetful mathematical skills, haha, and I dont want to waste time studying logs if they arent necessary (for acids and bases section).

 

[yalla22]

I can't understand Acids and Bases for my life!!

Specifically I have been looking in my EK book and have been getting a lot wrong related to "which one of the following is the strongest acid (or strongest base)?"

Here are some examples:

1. Which of the following is the strongest acid?
a.H20
b.H2S
c.H2Se
d.H2Te

2. Which of the following is NOT a strong base in aqueous solution?
a.LiH
b.CaH2
c.H2
d.KH

3. Which of the following statements is true?
I. NaH is a stronger base than NaOH
II. CaH2 is a stronger base than Ca(OH)2
III. HCl is a stronger acid than HClO

I don't know if I'm missing something basic or what-in fact, I'm pretty sure I'm missing something basic because I just dont know how to approach these problems w/o ka values. I also don't think it has anything to do with memorizing the strong acid/strong bases chart.


Any help would be greatly appreciated-thanks!!

 

[QofQuimica]

we need to know about logarithms and how to compute them. I have very forgetful mathematical skills, haha, and I dont want to waste time studying logs if they arent necessary (for acids and bases section).

Yes, you will need to know logs of base 10 and natural logs. However, you will not need to do very complicated calculations using logs, since you do not have a calculator on the MCAT. Here is a post about log rules.

 

[QofQuimica]

I can't understand Acids and Bases for my life!!

Specifically I have been looking in my EK book and have been getting a lot wrong related to "which one of the following is the strongest acid (or strongest base)?"

I don't have the EK book, but I'm guessing that they are trying to get you to use periodic trends to solve these problems. For example, in your first question, you'd predict that acidity should increase as you go down a group. If you check the pKa values, H2O is about 16, while H2S is about 7. I don't know what the other two are, but again, you'd predict that they'd be even more acidic. I suggest that you check for a section in your book on periodic trends for acidity and basicity, and that should help you with these questions.

 

[yalla22]

i guess another one of my problems is identifying bases-like I had no idea that N3- and like H- were bases? Or even Li2O?

 

[QofQuimica]

i guess another one of my problems is identifying bases-like I had no idea that N3- and like H- were bases? Or even Li2O?

If you're using the Bronsted-Lowry definition (most common one used), then anything that can accept a proton is a base. So yes, those species that you mentioned are all bases.

 

[yalla22]

i guess another one of my problems is identifying bases-like I had no idea that N3- and like H- were bases? Or even Li2O?

 

[Tracy47]

During corrosion, the metals get oxidized and the oxygen gets reduced. The best place to start when considering any problem like this is to write down a reaction for what is occurring. This will help you reason out each partner's role in the redox reaction. Consider this generic reaction that is occurring for an oxidation of a metal:

M (s) + O2 (g) -> M^xO^y

where x and y are positive integers. Your metal starts out in its standard state (solid metal), which has an oxidation number = 0. The same is true for the oxygen gas. After reaction to form the metal oxide, your metal is now a cation and your oxygen now has a formal charge of -2. Thus, the metal has been oxidized, and the oxygen has been reduced.

In the case of metals being dissolved in acid, the acid ends up forming H2 gas. Try to write an equation like we did before, and you should find that again the metal gets oxidized while the H gets reduced:

M + H+ -> H2 + M^n+

where n is some positive integer.

Thanks Q! Say if you're using HCl, what happens to the Cl-?

Are there any other generic redox reactions besides metal and oxygen, and metal and acids? Thanks so much!

 

[QofQuimica]

Say if you're using HCl, what happens to the Cl-?

It stays in the solution.

Are there any other generic redox reactions besides metal and oxygen, and metal and acids? Thanks so much!

There are tons of redox reactions. Burning a hydrocarbon is another common one:

CxHy + [x + (y/4)]O2 -> xCO2 + (y/2)H2O

where x and y are integers. Check out the electrochemistry and redox chapters of your gen chem textbook for more examples.

 

[Will Ferrell]

I was expecting this question to be asked already since it was brought up on the other forum, but i couldn't find it in the this thread. EK says that the volume of a real gas > V of an ideal and that the pressure of real gas < P ideal. There seems to be some disagreement about this. Is EK right?

Thanks!

 

[QofQuimica]

I was expecting this question to be asked already since it was brought up on the other forum, but i couldn't find it in the this thread. EK says that the volume of a real gas > V of an ideal and that the pressure of real gas < P ideal. There seems to be some disagreement about this. Is EK right?

Thanks!

It depends on how high the pressure is. You can read more about pressure and deviation from ideal behavior here.

 

[Twitch]

Hey Q -

I got another 100% on my CHEM test today (1/2 the class failed though) and wanted to say thanks for everything you're doing for us. If there is anything we can do for you please don't hesitate to ask.

Thanks,
-Y_Marker

 

[Tracy47]

Hi, I have a question about Fractional Distillation. Say I have sol A and B, and A has a higher boiling point. When I use a simple distillation column, does the receiver(where the gas condense) contain the liquid of higher boiling point, in this case, A? Thanks!

 

[QofQuimica]

Hey Q -

I got another 100% on my CHEM test today (1/2 the class failed though) and wanted to say thanks for everything you're doing for us. If there is anything we can do for you please don't hesitate to ask.

Thanks,
-Y_Marker

Nice job, Y_Marker! Since you mentioned helping, and we're getting close to MCAT time, we interrupt this gen chem thread for a public service announcement:

To those of you who are getting ready to take the MCAT, PCAT, DAT, or OAT soon and have found this forum helpful: If you do well on your test, please consider volunteering to help answer questions for future students on this forum. Previous teaching experience such as TAing or working for a test prep company is a plus but not necessary. If you're interested, you can contact Shrike or me about becoming a Q & A forum volunteer.

 

[QofQuimica]

Hi, I have a question about Fractional Distillation. Say I have sol A and B, and A has a higher boiling point. When I use a simple distillation column, does the receiver(where the gas condense) contain the liquid of higher boiling point, in this case, A? Thanks!

In general, the lower-boiling liquid tends to boil off first, condense first, and enter the receiving flask first. So you'll tend to have more of liquid A in the original flask, and more of liquid B in the receiving flask. The exception is if you have an azeotrope. You can read more about azeotropes here.

 

[HussainGQ]

1.Will you please explain to me the nernst equation and how it is related to the delta G, and the EMF of electric potentials in galvenic cells.

2. Will you also explain the concept of heat capacity, and how it is related to pressure and volume.

3. I understand that an increase in temperature means an increase in th k.E of a solution, but how does this kE avg.=3/2RT formula come into play?

 

[QofQuimica]

1.Will you please explain to me the nernst equation and how it is related to the delta G, and the EMF of electric potentials in galvenic cells.

2. Will you also explain the concept of heat capacity, and how it is related to pressure and volume.

3. I understand that an increase in temperature means an increase in th k.E of a solution, but how does this kE avg.=3/2RT formula come into play?

I will add all of those topics to the list for the Explanations thread. In the meantime, you can visit www.mcatpearls.com for help.

 

[hippocampus]

this question was in the physical science section, test 4r, number 35.

if a student did not remove all the moisture from the KHP before the titration with NaOH(aq), then the molarity determined for the NaOH(aq) would be...

answer: too high because the actual numbers of moles of KHP titrated would be less than the number used in the calculations

can you please explain this to me? i am confused...

 

[QofQuimica]

this question was in the physical science section, test 4r, number 35.

if a student did not remove all the moisture from the KHP before the titration with NaOH(aq), then the molarity determined for the NaOH(aq) would be...

answer: too high because the actual numbers of moles of KHP titrated would be less than the number used in the calculations

can you please explain this to me? i am confused...

NaOH is hygroscopic (absorbs water). This is why it is necessary to titrate NaOH with something like KHP in order to determine its exact molarity. KHP is not especially hygroscopic, but the KOH that is used to make it is. Maybe that was what the question meant? Anyway, if you weigh something hygroscopic on a balance and it is humid in the room, it will absorb some water from the atmosphere, and that will increase its weight above the true weight. For example, if I wanted 10 g of NaOH or KOH, and I didn't keep them dry, maybe my 10 g of pellets would be 9.5 g base and 0.5 g water. If I then calculated the moles of base using a weight of 10 g and didn't account for the water, then I would conclude that I have more moles of base than I really have. Thus, my calculated molarity would be too high as well.

 

[Blackstars]

Can someone please give me a detailed explanation for the following problems.

1. In an aqueous solution, the following equilibrium reactions are present.

Ag(NH3)2+ <--> Ag+ + 2NH3
NH3 + H2O <--> NH4+ + OH-
if the soluble salt, AgNO3 is added to the solution, the following changes in concetration takes place.

A: NH3, NH4+ and OH- decreases
B: NH3 increases, NH4+ and OH- decreases
C: NH4+ increases, NH3 and OH- decreases
D: NH4+ and OH- increases, and NH3 decreases
E: NH3 and OH- increases, NH4+ decreases.

2. If a solution which is initially 1.00M in compound X undergoes a decomposition reaction for 20.0 sec at an average rate of 0.020 mol/sec, what will be the new concentration of X

3. When 14.250 moles of Pcl5 gas is placed in a 3.00 Liter container and comes to an equilibrium at a constant temperature 40% of the pcl5 decomposes according to the equation PCl5 <--> PCl3 + Cl2. What is the value of Kc for this reaction
A: (1.896)^2/2.854 B: (2.854)^2/1.898 C: (3.800)^2/2.854 D: (2.854)(1.896)/3.800

4. In which solvent should NaCl be most soluble
A: CH3OH B: Octane C: diethyl ether D: CCl4 E: Benzene

 

[QofQuimica]

Can someone please give me a detailed explanation for the following problems.

Where are you stuck? Try to solve them yourself first, and then let me know if you need more help. The first problem is a Le Chatelier's principle problem. You can use dimensional analysis to do the second one. For the third one, calculate what the concentrations of your products are, and then substitute into the Keq expression. The last question asks you to determine which solvent would form the best ion-dipole interactions with a salt.

 

[medworm]

Alright, I'm kinda exhausted by reading and not finding a clearcut answer... could use some outside intelligence

I know that [conc] affects pH, but it seems to depend on the situation. Where do you draw the line? at 1M??

For instance, Q asks pH of 7M HCl soln = ?? Ans: Very straigforward, pH = -log [7] = -0.85.

But in another Q: Soln A has pKa =3.74 and Soln B has pka=4.74, which pH is higher? Ans: I can't tell the pH unless I have [conc]?? Why isn't pKa alone enough?

Confused

 

[QofQuimica]

Alright, I'm kinda exhausted by reading and not finding a clearcut answer... could use some outside intelligence

I know that [conc] affects pH, but it seems to depend on the situation. Where do you draw the line? at 1M??

For instance, Q asks pH of 7M HCl soln = ?? Ans: Very straigforward, pH = -log [7] = -0.85.

But in another Q: Soln A has pKa =3.74 and Soln B has pka=4.74, which pH is higher? Ans: I can't tell the pH unless I have [conc]?? Why isn't pKa alone enough?

Confused

Because pH depends on the concentration of H+ ions, but pKa depends on the RATIO of concentrations of dissociated ions to undissociated acid. Remember that Ka is the equilibrium constant for the dissociation of an acid:

HA <-> (A-) + (H+)

Ka = ([A-][H+])/[HA]

You can also re-write this as the Henderson-Hasselbach law, which may make the relationship between pH and pKa clearer to you. (See this post for the derivation of this equation):

pH = pKa + log [A-]/[HA]

Thus, if you know the pKa, you can get the ratio of products to reactants by taking the antilog. But you can't get the concentration of H+ unless you also know the concentration of HA. (You don't have to worry about the concentration of A-, since it will be the same as the concentration of H+ in most cases.)