Gen Chem Question

[dongkookLim]

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Accidentally posted it as a reply than a new thread...sorry about that.
That was really dumb...no wonder I can't do this problem... 🙁

From Practice Test.
What volume of HCl was added if 20 mL of 1 M NaOH is titrated with 1 M HCl to produce a pH = 2?
A. 10.2 mL
B. 20.2 mL
C. 30.4 mL
D. 35.5 mL
E. None of these.

Answer: E.
I'm not sure why. :-/ Thanks!

 

[mh0000]

Accidentally posted it as a reply than a new thread...sorry about that.
That was really dumb...no wonder I can't do this problem... 🙁

From Practice Test.
What volume of HCl was added if 20 mL of 1 M NaOH is titrated with 1 M HCl to produce a pH = 2?
A. 10.2 mL
B. 20.2 mL
C. 30.4 mL
D. 35.5 mL
E. None of these.

Answer: E.
I'm not sure why. :-/ Thanks!

1) Neutralize the base: NaVa = NbVb
Of course, you should just be able to look at the numbers and realize you will need 20ml of acid to nuetralize 20ml of the base since they are both at 1M and only contribute one acid-base component each (or basically, each one at 1 N).
This will leave us with 40 ml of neutral water (and salt). Now we need to keep adding acid to drop the pH to 2.

2) You need a pH of 2, which means [H+] = 0.01M. This basically becomes a dilution problem.
(1M)(x) = (0.01M)(x + 40 ml) Solve for x (the amount of additional acid used to get a pH of 2)

3) total amount of acid needed/added: x + 20