GC questions from destroyer

[Bigbirdo]

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These are the problems from the 2012 version.
1. Consider NO2, estimate the ONO bond angle
The answer is "over 120", but I chose "under 120". Because in the Lewis structure, the N is connected one oxygen via a single bond, another oxygen via a double bond, and have one non-bonded electron in the nitrogen. If a third bond is formed, the molecule will have 120 degrees bond angle. Since there are only two bonds and one electron, 120 degree angle should be suppressed a little, so I dont understand why it is over 120.

2. For question 193, why does the H3C6H5O7 dissociate to release three protons? As what I learned, the second and third dissociations are significantly small, so i think it would barely have effect on the molarity. The answer obviously takes the second and third proton into account. Can someone explain why I am wrong?

3. For question 42, the answer is A. Can you decide between A and E without knowing the charge of each? There is certainly no way we memorize all the multiple charges of these non-group 1 and 2 metals for the DAT.

4. For question 128, can you just multiple by 2 on the one side of the M1V1=M2V2 formula? I got the same answer. I do remember something from my gen chem class that my teacher told me not to use that, but I don’t remember why. What is the advantage of using normality for this question?

 

[supergenius310]

For 128 the general equation you use is NV = NV ....since Ca(OH)2 has TWO OHs you multiply the concentration in the equation by 2, so that's why you got the same answer.

and btw q 42 didn't show up on your pics

 

[Bigbirdo]

For 128 the general equation you use is NV = NV ....since Ca(OH)2 has TWO OHs you multiply the concentration in the equation by 2, so that's why you got the same answer.

and btw q 42 didn't show up on your pics

So what is the M1V1=M2V2 formula for?

Here is question 42

 

[yandillo]

Element As is in Group VA, recall that in this group N and P have 3- oxidation number...Plus Hg is one of those u should know that it has 1+ 2+ oxidation numbers ( as well as Cu and Hg).
For the other 2 questions u should always use normality. If u have HCl, HBr, which tells u that there is 1:1 ratio( this is the way I understand it) for this cases always N=M so u can use M1V1=M2V2,,,but when u have Ca(OH)2 there's no 1:1 ratio anymore,,,so u basically multiply your M for whatever is the subscript....recall that the subscripts can be in this position as well like in H2SO4,,,,so let's see that for the last 2 cases your Molarity was 3 then your Normality will be 6 for both cases. Hope u get it now.

 

[Jordwin]

I'm a little confused how the no2 bond angle is supposed to be greater than 120 as well. I would have thought it would have been less than 120.

 

[aznriptide859]

O will bond to N with both double bonds, so you will only have one lone electron attached to N. Normally with a pair of free electrons, you'll have a trigonal planar shape with 120 bond angles, but since there's only one electron, it requires less space and is exerting a smaller force on the two oxygens. Therefore the oxygens will push out a bit more and have a bond angle greater than 120 (I think Wiki lists it at 134?).

 

[Jordwin]

Ahh okay thanks that makes sense now!

 

[Piepiesuperpie]

I think it's because there is resonance and if you drew the hybrid structure out it'd be double partial bonds to both O. In which case the angle would be between 120-180.

http://www.madsci.org/posts/archives/2000-02/951452623.Ch.1.gif

 

[sskater]

For #2(193).

We see that when we get the normality of Citric acid, it is 0.48N.
Divide by 3 because of the 3H+ dissociating(more on this later), and we get 0.16.
We see that the answer B? is 0.167, which is slight more than our calculated answer.
This makes sense in regards to your reasoning in which the 2nd and 3rd protons contribute less to the soln.
If we divided 0.48/3=0.16, but if we divided it by 0.48/2.8, we get ~0.167. So the 3rd proton did contribute less to the solution. Hope this helps.