General Chemistry Question Thread

[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

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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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[medworm]

I often get the signs confused... can someone please verify for me that:
+ endothermic for BDE, IE, enthalpy, heat of solvation.
- exothermic for BDE, IE, EA, lattice energy.

Somewhere in my notes, I have "to form strong bonds", lattice E > solvation E for hydration". How can negative lattice E be > than positive solvation E?

Is lattice E ever + endo?

Thanks so much.

 

[QofQuimica]

I often get the signs confused... can someone please verify for me that:
+ endothermic for BDE, IE, enthalpy, heat of solvation.
- exothermic for BDE, IE, EA, lattice energy.

Somewhere in my notes, I have "to form strong bonds", lattice E > solvation E for hydration". How can negative lattice E be > than positive solvation E?

Is lattice E ever + endo?

Yes, endothermic processes should have a (+) sign, and exothermic processes should have a (-) sign. I'd recommend that you not simply blindly follow sign conventions, however. Always stop and consider whether an answer is reasonable; this will help you avoid making careless mistakes with the sign conventions in both gen chem and physics.

Lattice enthalpy measures how strongly a crystal is held together by electrostatic forces between the cations and anions. Solvation enthalpy measures the attraction of the solvent molecules for the individual ions of the crystal. If the magnitude of the lattice energy is greater than that of the solvation energy, the crystal will not dissolve. You could interpret this to mean that the ionic bonds in the crystal are "too strong" for it to dissociate into its constituent ions, at least under those conditions.

 

[GCT]

I often get the signs confused... can someone please verify for me that:
+ endothermic for BDE, IE, enthalpy, heat of solvation.
- exothermic for BDE, IE, EA, lattice energy.

Somewhere in my notes, I have "to form strong bonds", lattice E > solvation E for hydration". How can negative lattice E be > than positive solvation E?

Is lattice E ever + endo?

Thanks so much.

adding to what QofQuimica already said

the standard enthalpy for the step pertaining to lattice energy formation is always negative (review Hess' law). You should remember that frequently, almost always in your case, bond formation itself releases "energy" while bond breakage requires "energy" (strictly in reference to bonds). In the case you mentioned, dissolution, refers to bond breakage/dissociation; it refers to the reverse process of lattice energy thus it's the negative of this value (positive). You should review Hess' law in light of this question-dissolution requires the break up of the lattice, than solvation/hydration by water.

I think that you may need to review the basics a bit more, try going over the thermodynamic aspects in a standard general chemistry text. Although the asks simple questions, they can get nasty at times and trip you up (never taken the official mcat though). There actually so much more to thermodynamics, even at the gen. chem level. As QofQuimica said, it won't suffice to simply memorize, thus MCAT reviews are frequently inadequate, you can't apply thermodynamics perfunctorily, it's not so clear cut and simple.

 

[QofQuimica]

I made your posts into a new thread in the main MCAT forum.

P.S. Are you from Toronto?

 

[stoleyerscrubz]

how does one quantitatively distinguish between a saturated and supersaturated solution?

 

[Nutmeg]

how does one quantitatively distinguish between a saturated and supersaturated solution?

If the concentration is higher than the saturation level, then it is a supersaturated solution. Such a solution is unstable and can quickly produce a precipitate, but momentarily one can get a solution with more disolved material than the numbers indicate as being possible.

 

[GCT]

how does one quantitatively distinguish between a saturated and supersaturated solution?

K...s....p, solubility constant, can't explain it all here though, you'll need to read up on it. If you have any specific questions or run into a problem, feel free to ask.

 

[QofQuimica]

For some reason, I can't answer PMs, so I am going to post this answer to you here, and hopefully you will see it:

*********

Your question looks scarier than it really is because of the equation. What you need to do is figure out where the variables are, and you can ignore the rest of the letters, which are all constants. In this case, Za and Zb are going to be the variables. I and Ea will both be constants, as will everything else in the third term. You want to have a bond, which means that you need a positive value for D. The only way to get it is for I + Ea + ZaZb to equal a positive number. I is always negative, Ea is always positive, and Za is always positive, but Zb is always negative, which means that you will get a positive number for term 3 once the ions start forming. When that number gets large enough, the sum of term 3 and Ea will overtake the negative value of I, meaning that you get a positive value for D, and you get your bond.

 

[N1DERL&]

Thanks Q! That makes so much more sense than the explanation.

Could you explain bond dissociation energy? What does a positive vs. negative BDE mean?

I also have another easy question for you... It's about calculating pH:
You have 1 mL of a strong acid pH 2.3
It's diluted with water to make 100 mL.
What is the new pH?

 

[vmp200]

I also have another easy question for you... It's about calculating pH:
You have 1 mL of a strong acid pH 2.3
It's diluted with water to make 100 mL.
What is the new pH?

not sure if this is correct, but wouldnt the pH be 4.3.
M*V = M*V
5*10^-3 * 1mL = 100mL * M
M = (5*10^-3)/100 = 5*10^-5 = 4.3
????

 

[QofQuimica]

Could you explain bond dissociation energy? What does a positive vs. negative BDE mean?

Check further up the page; there are some other posts about this.

I also have another easy question for you... It's about calculating pH:
You have 1 mL of a strong acid pH 2.3
It's diluted with water to make 100 mL.
What is the new pH?

pH is -log [H+] Your moles of H+ don't change, but your molarity does, b/c your volume changes. See what the poster between us wrote.

 

[HailToTheThielf]

can anyone explain the equation G=-lnKeq Thnaks guys, appreciate it!

 

[QofQuimica]

can anyone explain the equation G=-lnKeq Thnaks guys, appreciate it!

See if this post helps. I am going to write an explanations post about thermochemistry soon, since it keeps coming up.

 

[05med05]

Hi,

The instructors from my mcat class seem to be mising in action (even though they said that they would be available until the morning of test day (Heh, heh).

Anyhow I have a few chem questions / concepts that I that I'm mixed up on, so here goes:

Q# 23 of PT8 (or AAMC 8R)

M(OH)2 has a molar solubility of S. What is the KSP?

THe anwser was 4S cubed. Now I know KSP = [M] [OH]2, so why isn't the answer 2S squared, ie) why is KSP= [2S]2 instead of 2??
--------
2nd Q (I should know this, but...)
For Glavanic vs Electrolytic cells, if we have the following Galvanic cell:

Pt / Zn(s) / Zn2+ / Cu2+ / Cu(s) / Pt, then for the corresponding electrolytic cell, we would apply a voltage and the Zn would become the cathode and Cu, the anode, ie just reverse the direction of the electron flow so that the Cathode is - and the anode + ??

-------------

3rd Q
In PV=nRT, which would result in a greater change in temp, a change in volume at contant pressure, or a change in pressure at constant volume?

-----------------

Last Q
Does pKb follow the same rules as pKa? ie)

Pkb of a strong base < pKb of a weak base.

Also, there was a question in an old Kaplan exam (I can't remember which one), but, it was talking about the pkb of HCl. Don't they really mean pKb of Cl-, because HCl is the acid and Cl- the conjugate weak base?

Or do HCl and Cl- each have their won corresponding pkb values? (I'm trying to relate this to Water which has pka +pKb = 14.

Any help would be greatly appreciated!!!
Thanks

 

[hippocampus]

if Q > Ksp, a ppt form..

but what about le chat's principle? wouldn't it make it not form?

so does it ppt, or does it not?

thanks!

 

[QofQuimica]

Q# 23 of PT8 (or AAMC 8R)

M(OH)2 has a molar solubility of S. What is the KSP?

THe anwser was 4S cubed. Now I know KSP = [M] [OH]2, so why isn't the answer 2S squared, ie) why is KSP= [2S]2 instead of 2??

Because when the salt dissociates, it forms TWO moles of ions for every mole of salt, not just one. That is, if you have x moles of salt, that will give you x moles of M, and 2x moles of S. So that coefficient two designates that you form 2 moles of anion per mole of salt. You square the whole 2x because you are trying to figure out the equilibrium constant (Ksp is a type of Keq). So the exponent is there because you have to multiply the concentration of each ion together to get the Keq, and if you multiply 2x * 2x, you end up with 4x^2.

I recommend that you use a table to solve Ksp problems. It will help you keep yourself organized so that you don't mess up the math.

2nd Q (I should know this, but...)
For Glavanic vs Electrolytic cells, if we have the following Galvanic cell:

Pt / Zn(s) / Zn2+ / Cu2+ / Cu(s) / Pt, then for the corresponding electrolytic cell, we would apply a voltage and the Zn would become the cathode and Cu, the anode, ie just reverse the direction of the electron flow so that the Cathode is - and the anode + ??

Yes.

3rd Q
In PV=nRT, which would result in a greater change in temp, a change in volume at contant pressure, or a change in pressure at constant volume?

If you change P and V by the same amounts (ex. double pressure or volume), the temperature will change by the same amount in each case. n and R are constants, and the product of P * V will be the same in both cases, regardless of whether it is (2P) * V, or P * (2V)

Last Q
Does pKb follow the same rules as pKa? ie)

Pkb of a strong base < pKb of a weak base.

Yes.

Also, there was a question in an old Kaplan exam (I can't remember which one), but, it was talking about the pkb of HCl. Don't they really mean pKb of Cl-, because HCl is the acid and Cl- the conjugate weak base?

Yes. HCl is not able to act as a base; it isn't amphoteric like water is.

 

[QofQuimica]

if Q > Ksp, a ppt form..

but what about le chat's principle? wouldn't it make it not form?

so does it ppt, or does it not?

It does precipitate. You have to be careful about applying Le Chatelier's Principle to salts. If you already have a saturated solution, adding more salt will *not* cause the equilibrium to move toward more salt dissolving. No more salt will dissolve; that's what it means to say that the solution is saturated! In that case, what happens is that the extra salt simply sinks to the bottom as ppt.

 

[hippocampus]

if Q > Ksp, a ppt form..

but what about le chat's principle? wouldn't it make it not form?

so does it ppt, or does it not?

thanks!

 

[lropp]

Using the water densities at 20 and 35 °C,
D20 = 0.9982 g/cc D35 = 0.9941 g/cc
APPROXIMATE by interpolation the density at 29.1 °C


PLEASE HELP!

 

[Arsenic]

Using the water densities at 20 and 35 °C,
D20 = 0.9982 g/cc D35 = 0.9941 g/cc
APPROXIMATE by interpolation the density at 29.1 °C


PLEASE HELP!

http://core.ecu.edu/chem/chemlab/appendix/appendixC.htm

qofquimica will probably merge these posts into another thread or something when she see's them, i'll leave that for her to do since this forum is her baby.

*EDIT*

at 16 minutes, hope that was ASAP enough

 

[QofQuimica]

http://core.ecu.edu/chem/chemlab/appendix/appendixC.htm

qofquimica will probably merge these posts into another thread or something when she see's them, i'll leave that for her to do since this forum is her baby.

*EDIT*

at 16 minutes, hope that was ASAP enough

Merging this thread with the Gen chem thread. Guess I will let you take over my forum, Arsenic; you're doing a great job.

Iropp, please don't start new threads in the subforum. Just post your questions in the appropriate thread for that subject.

 

[lropp]

http://core.ecu.edu/chem/chemlab/appendix/appendixC.htm

qofquimica will probably merge these posts into another thread or something when she see's them, i'll leave that for her to do since this forum is her baby.

*EDIT*

at 16 minutes, hope that was ASAP enough

THANK YOU!

 

[Truth_seeker]

Good evening. I've got a question on my solubility homework for lab. I found that precipitate reactions occured using Mg+2 cation and OH-1, CO3 2-, and PO4 3- anions. The question I've been getting wrong is asking me to write a balanced net ionic equation for these three precipitate reactions. What I've got so far is(where ^# equals the ionic charge) Mg^2 + 2(OH^1-) ->Mg(OH)2, Mg^2 + CO3^2- -> MgCO3, and 3Mg^2 + 2(PO4^3-)2. However, my assignment on the web says it's wrong each time. HELP. Thanks for any advice on figuring this out.

 

[Nutmeg]

Good evening. I've got a question on my solubility homework for lab. I found that precipitate reactions occured using Mg+2 cation and OH-1, CO3 2-, and PO4 3- anions. The question I've been getting wrong is asking me to write a balanced net ionic equation for these three precipitate reactions. What I've got so far is(where ^# equals the ionic charge) Mg^2 + 2(OH^1-) ->Mg(OH)2, Mg^2 + CO3^2- -> MgCO3, and 3Mg^2 + 2(PO4^3-)2. However, my assignment on the web says it's wrong each time. HELP. Thanks for any advice on figuring this out.

I think you just typed it in to the computer wrong. You have a "2" in front and behind the PO4 in the last bit, and you haven't written it all out (ie there's no arrow on the last one).

The last should be:

3Mg^2 + 2(PO4^3-) --> Mg3(PO4)2

But other than that, the first two are fine.

 

[medstu2006]

What is the difference between a kinetic assay and an endpoint assay?

 

[QofQuimica]

What is the difference between a kinetic assay and an endpoint assay?

An endpoint assay is where a reaction is run for some period of time, and you measure the amount of substrate converted to product at the end of that time. In contrast, the concentrations of products are measured during the course of the reaction in a kinetic assay. Note that this question is beyond what you would need to know for the MCAT.

 

[suckermc]

confused about this.

i have a weak acid (HCN) and i want to find the pH of a .01 solution.

okay so i set up the equation

H+/CN over HCN.

then i set it up like

x x /.01=6.2x10-10.

how is the PH between 5 and 6?

 

[aimsz88]

hi guys,

this question would seem easy but i got it wrong while practicing using EK.

q: which of the following has naturally occurring ions with two different charges?

i narrowed the answers down to V and H...but I chose H because there is the hydride ion H-, as well as the more common H+. the answer was actually V, because it is a transition element with more unstable d shells.

where did i go wrong? any help? thanks!!

 

[QofQuimica]

hi guys,

this question would seem easy but i got it wrong while practicing using EK.

q: which of the following has naturally occurring ions with two different charges?

i narrowed the answers down to V and H...but I chose H because there is the hydride ion H-, as well as the more common H+. the answer was actually V, because it is a transition element with more unstable d shells.

where did i go wrong? any help? thanks!!

That's a pretty ambiguous question; I don't think your reasoning is bad, actually. But evidently, they were looking for you to recognize that many transition metals can take on multiple cationic states due to their being able to lose different numbers of d electrons. On the real MCAT, you should never have an ambiguous question that has two possible answers like that. This question could be reworded: Which of the following elements can have more than one naturally occuring CATION with different charges? And then it would be clear that the transition metal was the correct answer.

 

[PRamos]

confused about this.

i have a weak acid (HCN) and i want to find the pH of a .01 solution.

okay so i set up the equation

H+/CN over HCN.

then i set it up like

x x /.01=6.2x10-10.

how is the PH between 5 and 6?

Your set up is exactly correct. Considering x to be H+ concentration, you get x x/0.01 = 6.2x10-10

From here, you need to multiply both sides of the equation by 0.01, to get rid of the denominator. This leads to:

x x = 6.2x10-12

The square root of 6.2x10-12 is the [H+], which is about 2.5 x 10-6.

This means that the pH is 6 - log 2.5, which is about 5.6

• There is a really easy shortcut I learned in my MCAT prep class, but I'm not sure if those can be discussed here.

 

[PRamos]

hi guys,

this question would seem easy but i got it wrong while practicing using EK.

q: which of the following has naturally occurring ions with two different charges?

i narrowed the answers down to V and H...but I chose H because there is the hydride ion H-, as well as the more common H+. the answer was actually V, because it is a transition element with more unstable d shells.

where did i go wrong? any help? thanks!!

Don't let those questions phase you. I found that when I did EK books, there were many of those types of ambiguities. You are actually correct if the question is taken literally. H can exist in the two states you mentioned. V can exist as a 2+, 3+, or 5+ ion, so it actually has three cationic forms.

But the books are a learning tool, and the key point is that transition metals often have mulitple ionic states.

I'm not sure if you find this to be true, but because the EK chemistry books were not written by a chemist, they have a bunch of little errors like that. Nothing critical, but it got annoying at times. I kind of found the same thing with their other books too, but I got used to it.

 

[QofQuimica]

bump

 

[suckermc]

is steady state/equilibrum approximation going to be on the mcat? what about michealis (enzyme kinetics?) im using so many review books and one discusses it, tpr doesnt, so do i need to know this?

 

[QofQuimica]

is steady state/equilibrum approximation going to be on the mcat? what about michealis (enzyme kinetics?) im using so many review books and one discusses it, tpr doesnt, so do i need to know this?

If it shows up, it will be on a very basic level, and most likely associated with a passage. You may be asked some simple biochemistry questions on the MCAT, but only about topics that are covered in your freshman gen chem and sophomore organic courses. If you haven't already, take a look at the AAMC's list of topics that could appear on the MCAT. There is a link to it on the opening post of this thread.

 

[suckermc]

If it shows up, it will be on a very basic level, and most likely associated with a passage. You may be asked some simple biochemistry questions on the MCAT, but only about topics that are covered in your freshman gen chem and sophomore organic courses. If you haven't already, take a look at the AAMC's list of topics that could appear on the MCAT. There is a link to it on the opening post of this thread.

you know as i was waiting for a response i did check the list of topics...i noticed that EK talks about reversable reactions and TPR did not and it reminded me of bio lab and enzyme kinetics, so i freaked. but thanks!

 

[Lindyhopper]

Hi I have a practice question that asks which answer chioce would have the least impact on a saturated solution of NH3 (aq).

A. HNO3
B. I2
C. N2O5
D. NH4OH

D. is the answer but no explaination is given. I assume that NH4OH is correct for the following reason. NH3 is a base. In aqueous solution the equilibrium would be HOH + NH3 < -> NH4+ OH-. So there would be no significant impact as we will just be increase the amount of what we have.

1. Is this reasonable?
2. I have no idea how the other three choices would react. Can you offer any insights?
3. How does one do subscripts on SDN?
Thanks

 

[QofQuimica]

Hi I have a practice question that asks which answer chioce would have the least impact on a saturated solution of NH3 (aq).

A. HNO3
B. I2
C. N2O5
D. NH4OH

D. is the answer but no explaination is given. I assume that NH4OH is correct for the following reason. NH3 is a base. In aqueous solution the equilibrium would be HOH + NH3 < -> NH4+ OH-. So there would be no significant impact as we will just be increase the amount of what we have.

1. Is this reasonable?
2. I have no idea how the other three choices would react. Can you offer any insights?
3. How does one do subscripts on SDN?
Thanks

1. Yes. NH4OH is the same thing as aqueous NH3.
2. HNO3 is an acid and would neutralize the solution. Not sure about the other two. I think B may be the way to make nitrogen triiodide.
3. As far as we can tell, there isn't any way. If anyone knows of a way, I'd like to know how to do it too!

 

[Lindyhopper]

I can't quite get a handle on two different restatements of the 2nd law of thermodynamics. I think I understand the Carnot machine & how it shows that heat can not be completely converted into work in a cyclic process. I don't see how this is requires all real reactions to increase the entropy of the universe.

 

[Lindyhopper]

Sorry. Double post.

 

[Lests55]

In my Princeton book there is a question that makes you choose the quantum numbers for the electron furthest from the Ni nucleus. My reasoning was that it had to be the 4s electrons because of the n values (since n=distance from nucleus) but the answer was the 3d. I understand that the 3d are higher in energy (filled up first) but still thought that the 4s would be FURTHER from the nucleus.

Thanks in advance.

Q, (on a side note) have you narrowed down your choices for where you want to go yet? (just curious!)

 

[Lindyhopper]

Delta G is "the energy that's free to do useful work from a chemical reaction." Can you possibly suggest a way to understand this. What would "Non free" energy be.
Thanks,

 

[DrChandy]

Delta G is "the energy that's free to do useful work from a chemical reaction." Can you possibly suggest a way to understand this. What would "Non free" energy be.
Thanks,

First, think of free energy as being extra energy.

----------------------------------------------------

There is energy in atomic bonds. There is also energy in temperature and entropy. [S=Q/T, therefore [B]Q=SxT[/B]] In a chemical reaction, heat from temperature and entropy need to be enough to excite and break the bonds of atoms.

----------------------------------------------------

Keeping the above in mind, when the energy from the product of [temperature and entropy] are greater than the bond energies of the atoms involved in a chemical reaction, there is energy left over. This "free energy" can be used to do work.

Non-free energy would be energy that is stored in the bonds of the atoms of the chemicals in the reaction, which cannot be liberated to do work, because heat and entropy [i.e. the product of which] are too low.

 

[DrChandy]

In my Princeton book there is a question that makes you choose the quantum numbers for the electron furthest from the Ni nucleus. My reasoning was that it had to be the 4s electrons because of the n values (since n=distance from nucleus) but the answer was the 3d. I understand that the 3d are higher in energy (filled up first) but still thought that the 4s would be FURTHER from the nucleus.

Thanks in advance.

Q, (on a side note) have you narrowed down your choices for where you want to go yet? (just curious!)

Note that the higher the energy of an electron, the further away it is from the nucleus.

Make sure you go over electron orbital filling in order to answer this type of question.

 

[QofQuimica]

I can't quite get a handle on two different restatements of the 2nd law of thermodynamics. I think I understand the Carnot machine & how it shows that heat can not be completely converted into work in a cyclic process. I don't see how this is requires all real reactions to increase the entropy of the universe.

Real reactions will increase the entropy of the universe because they are not 100% efficient. The energy that gets "wasted" in any process (i.e., given off as heat) is what increases the entropy of the universe.

 

[QofQuimica]

In my Princeton book there is a question that makes you choose the quantum numbers for the electron furthest from the Ni nucleus. My reasoning was that it had to be the 4s electrons because of the n values (since n=distance from nucleus) but the answer was the 3d. I understand that the 3d are higher in energy (filled up first) but still thought that the 4s would be FURTHER from the nucleus.

Thanks in advance.

Q, (on a side note) have you narrowed down your choices for where you want to go yet? (just curious!)

Comparing the sizes and energies of the 4s and 3d orbitals is often a confusing issue for students. You are correct that in general, orbitals of a higher quantum number should be a greater distance away from the nucleus. This would definitely be true if you were comparing two like orbitals, say, 4s versus 3s. However, things get more complicated when you are considering different kinds of orbitals, because electrons have different levels of penetrance (ability to be near the nucleus) depending on the type of orbital they are in. Thus, a 3s electron will spend more time near the nucleus compared to a 3p one, which in turn has more access to the nucleus than a 3d electron. As you get into orbitals with more nodes (places where electron density is zero near the nucleus; recall the the shape of an s-orbital is spherical, while the others all have at least one node around the nucleus), the electrons in those orbital types spend more and more time farther away from the nucleus. This occurs at every quantum level.

So now you can see that you have two competing factors here: on one hand, 3 is a lower quantum level than 4, so you would reasonably predict that the n = 3 electrons would be closer to the nucleus than the n = 4 electrons. That is true for the 3s and 3p electrons. However, you have the counteracting effect that d electrons have much less penetrance than s electrons. In this case (3d versus 4s), the balance works out where the 4s electrons' greater penetrance overcomes the higher primary quantum number factor, and so they are on average closer to the nucleus than the 3d electrons are even though their principle quantum number is one higher. Does this help?

In answer to your other question, no, not yet.

 

[warren]

Hi everyone. I am new here and I want to post a question relating to ideal gasses. How do I create a new thread and post my question. Thanks in advance.

Warren

 

[QofQuimica]

Hi everyone. I am new here and I want to post a question relating to ideal gasses. How do I create a new thread and post my question. Thanks in advance.

Warren

Hi Warren,

Please don't create a new thread in this subforum. And please don't post in the explanations thread. You can just ask your question in the already existing Gen Chem thread. I am going to move these two messages to that thread now, and you can feel free to post your question there.

 

[ichimaru]

We just reviewed VSEPR in our class today, and there were several shapes I had forgotten, like the bent shape, see-saw, square pyramid, t-shape, and square planar (p. 363 of the 2006 TPR book). Our teacher recommended that we memorize all of those, but have they ever surfaced on the MCAT? I'm not sure if it's worth spending time on these.

My university G.Chem professor only made us learn the basic geometry (linear, trigonal planar, tetrahedral, trigonal bipyramid, & octahedral).

 

[QofQuimica]

We just reviewed VSEPR in our class today, and there were several shapes I had forgotten, like the bent shape, see-saw, square pyramid, t-shape, and square planar (p. 363 of the 2006 TPR book). Our teacher recommended that we memorize all of those, but have they ever surfaced on the MCAT? I'm not sure if it's worth spending time on these.

My university G.Chem professor only made us learn the basic geometry (linear, trigonal planar, tetrahedral, trigonal bipyramid, & octahedral).

I would know the bent shape, since water is such an important compound. But you can probably forget about the see-saw, square pyramid, t-shape, and square planar shapes. You should also know the trigonal pyramidal (ex. ammonia) and the basic geometries that you already listed.

 

[Mr. Three-Wiggle]

Here's a heat transfer question for ya. This is from memory but here goes:
you've got a 50g piece of metal and heat it up to 100 degrees C. The metal is put in 50g of water starting at 25 degrees C. What's the final temperature of the water? You're given the c of the metal: 25 - also it's in units such that c for water is 1.

so I set it up like this: (50)(.25)(Tfinal-100) = (50)(1)(Tfinal-25)

the answer has it set up such that the delta Ts are reversed ie. (100-Tfinal) and (Tfinal - 25).

I guess I just don't understand why the delta T of the metal initial minus final. If someone could clarify this for me that'd be killer - thanks.