AP chem
posted by Corey on .
This is part of a marathon problem, we have to identify the five compunds of H, N, and O described below. For each compound, I have to write a Lewis structure that is consistent with the information given.
a.) All the compunds are electrolytes, although not all of them are strong electrolytes. Compunds C and D are ionic and compund B is covalent.
b.) Nitrogen occurs in its highest possible oxidation state in compunds A and C; nitrogen occurs in its lowest oxidation state in compunds C,D, and E. The formal charge on both nitrogen in compund C is +1; the formal charge on the only nitrogen in compund B is 0.
c.) Compund A and E exist in solution. Both solutions give off gases. Comercially available concentrated solutions of compund A are normally 16 M. The commercial, concentrated solution of compound E is 15 M.
d) Commercial solutions of compund E are labeled witha misnomer that implies that a binary, gaseous compund of nitrogen and hydrogen reacted with watert to produce ammonium ions and hydroxide ions. Actually this reaction occurs to only a slight extent.
e) Compound D is 43.7% N and 50.0% O by mass. If compound D were a gas at STP, it would have a density of 2.86 g/L.
f) A formula unit of compund C has one more oxygen than a formula unit of compound D. Compounds C and A have one ion in common when compound A is acting as the strong electrolyte.
g) Solutions of C are weakly acidic; solutions of compound A are strongly acidic; solutions of compound B and E are basic. The titration of .726 g of compound B requires 21.98 ml of 1.000 M HCl for complete neutralization.
This is due tomm and I have no idea what the question is asking or how to start.

Start with e).
You know N is 43.7%, O is 50.0%, so H must be 100%  those two. Calculate the empirical formula from this. Knowing the density of 2.86 (if it were a gas) calculate the molar mass, then determine the molecular formula.
Now go to f).
g) should give you the molar mass of comound B.
The rest of it then falls into place.
Post your work and we can go from here if you need additonal hints. (How long have you had this marathon problem if it is due tomorrow?) 
Well i tried to get the empirical formula but i got H2NO, how do i use the density, 2.86.

H2NO is correct. Now if it were a gas (it isn't) the density would be 2.86. Remember that 1 mol of a gas occupies 22.4 L atr STP. So the molar mass of this material is ??. And that means the molecular formula is ??

I am not sure how to find the molar mass using the 22.4 L at STP

2.86 g/L x 22.4 L/mol = ??

64.064 g/mol

So is H2NO correct?

No. What's the molar mass?

the molar mass is 64.064 g/mol

Now what is the formula mas of the empirical formula you worked out? So now you know the molecular formula is ??

it should be H2NO3

Come on now. You have the molar mass correct at 64. You know the emprical formula is H2NO. So what is the formula mass of the H2NO (that's the emprical formula).All you need to do is add the atomic masses.

Ok so i am done with D

Because when i add the atomic masses i dont' get 64

You are getting ahead of yourself. What is the formula mass of the empirical formula. And what is the molecular formula. (Or tell me what you don't understand).
So what is D. 
the formula mass is 32

Good. So if the formula mass is 32 and the molar mass is 64, that means the molecular formula must be ??

NH4NO2

VERY GOOD.
So D is NH4NO2. Now you know what C is? It is ??(from statement f) 
C would be NH4NO3

RIGHT
Next I would go to statement g and work out the molar mass of compound B. 
Ok but i am confused of how to use the info for B

mols cmpd B = L x M
mols = g/molar mass 
I got the mols compound B = .002198 when i did the 21.98ml * 1M

Is this correct? the molar mass of B is 330.3

Hey i got the molar mass of B to be 330

Surely not. I think you have a decimal point off.
mols = L x M = 0.02198 L x 1.000 M = 0.02198
molar mass = g/mols = 0.726/0.02198= xx 
oh ok my bad so the molar mass is 33.03

so now how do i find the formula

ok. Now you know the molar mass is about 33, you know from statement b that compound B has 1 N atom (with a formal charge of zero) AND from statement g that compound B is basic. So you need to put 1 N, an OH, and some H atoms to make 33.

would it be NH2OH

Probably, although you can't be definite about it with just a molar mass, 1 N, basic, and formal charge of zero. Those things fit but to be defnitive you would need an elemental analysis like you went through for compounds C and D.
OK.
Do you know the name of H2NOH?
Now you know B, C, and D. That leaves A and E. Not enough information is available about A or E to be definitive but you can make VERY good guesses about their identity. 
would A be HNO3 and E be NH3

That's what I would guess. Statement c is a dead give away. I know HNO3 is sold commercially as 16 M solution and that NH3 is sold commercially as 15 M. Also, I know the bottle is labeled ammonium hydroxide and not aqueous NH3. So these probably are correct. The other statements are consistent with A and E being HNO3 and NH3.
I trust you can do the Lewis dot structures. 
yes i can do them so i don't need to do any work for A and E?

Only to show that they are consistent with statements a, b, c, d, and g

Ok thanx alot i appreciate it

Dr. Bob yur a beast:]

i had the same problem for ap chemistry and i looked at what you wrote but could you help me with it

Whoever did this, thank you so much! You just saved my life on this problem and on my next test because it was extra credit and now I actually understand how to do it.

Thanks!

Thanks for working everything stepbystep! Truly appreciate it!