TopScore 2: few questions

[pbure9]

I had a couple of questions from the 2nd topscore test.

First question is from chem.. number 68 (with how much 1M HCl you have to add 20mL of 1M NaOH to get a solution with ph=2). So I know that means you need .02M H+. Anyway, if someone can show me how its done I'd appreciate it.

Also, 74 from orgo. The NMR with m-xylene. So I know its symmetrical. I got the answer right because I knew it had to have only 4 distinct Hydrogens, but, my problem is that I thought it would be 2 singlets and 2 doublets. I know symmetrical H's split once, so would the H I circled in my attachment see both H's or just 1 and be a doublet?

Thanks guys!

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[nze82]

I had a couple of questions from the 2nd topscore test.

First question is from chem.. number 68 (with how much 1M HCl you have to add 20mL of 1M NaOH to get a solution with ph=2). So I know that means you need .02M H+. Anyway, if someone can show me how its done I'd appreciate it.

pH = 2 --> [H+] = 10^-2 = 0.01M

M1V1 = M2V2
(1M)(x) = (20ml)(1M)
x = 20ml of HCl is required to completely neutralize the base.
After eliminating all the OH- species within the solution, we must add enough 1M HCl, so that [H+] = 0.01M

(1M)(xml) = (20+xml)(0.01M)
x = 0.2 + 0.01x
x - 0.01x = 0.2
0.99x = 0.2
x = 0.2ml

So, the total volume of HCl you've added is 20ml(to neutralize the base) + 0.2ml(to bring the pH down to 2) = 20.2ml of HCl

Also, 74 from orgo. The NMR with m-xylene. So I know its symmetrical. I got the answer right because I knew it had to have only 4 distinct Hydrogens, but, my problem is that I thought it would be 2 singlets and 2 doublets. I know symmetrical H's split once, so would the H I circled in my attachment see both H's or just 1 and be a doublet?

Thanks guys!

Hope this helped!

 

[UCB05]

And for your second question, m-xylene will have a singlet from the methane substituents, a singlet from the hydrogen on the carbon between the two methyl carbons and a doublet from the two hydrogens on the other side of the carbons, split by the hydrogen attached to the carbon in the meta position. The hydrogen attached to the meta carbon will be a triplet because there are two non-equivalent-to-self hydrogens, one on each carbon flanking it. Remember all non-equivalent hydrogens add another peak to a signal, even if theyre on separate carbons but electrically equivalent to each other.

 

[pbure9]

And for your second question, m-xylene will have a singlet from the methane substituents, a singlet from the hydrogen on the carbon between the two methyl carbons and a doublet from the two hydrogens on the other side of the carbons, split by the hydrogen attached to the carbon in the meta position. The hydrogen attached to the meta carbon will be a triplet because there are two non-equivalent-to-self hydrogens, one on each carbon flanking it. Remember all non-equivalent hydrogens add another peak to a signal, even if theyre on separate carbons but electrically equivalent to each other.

alright thanks man. I was just wondering if symmetry splits it differently