[bettyyy]

I had this on my test today, and I'm trying to see if I got it right.  4 of my classmates all got different answers.

it's a 2 part question.
What is the pH of .005 M HBr? pH=2.30
What is the pH of .01 M KOH?  pH=12.00

If you combined equal parts of these solutions together, what would the pH be?

Please help me!

[Borek]

This is basically a limiting reagent question.

Show how you tried to solve it.

[bettyyy]

I'm pretty sure I got it wrong, I got confused where it said equal volumes, so I wrote something like:

Kw=[OH][H]
H + OH => H20
Kw=X^2
1.0 x 10^-14=x^2
so therefore pH = 7 because OH and H would neutralize each other.

But, I'm sure molarity had effect on the answer.

[Borek]

Equal volumes - assume you mixed 100 mL of one with 100 mL of the second. What should have happen?

[bettyyy]

okay, I think I've gotten it.  Because equal amounts were added, I should of picked a number to figure out how many mL it would take to reach equilibrium. 

The question doesn't state if the acid or the base is being titrated... so I am going to guess I'm going strong base into a strong acid.  So I would need to figure out how many mL of HBr to reach equil.

If it were to be 100 mL:
(100mL)(.01)=(.005)(Ve)
1 mL = .005Ve
Ve=200 mL

So you would need 200 mL of .005 M HBr to titrate 100 mL of .01 M KOH.  Because we only have 100 mL, we would be left would a more basic solution.

[Borek]

I think you are overcomplicating, even if you are right that final solution will be basic.

As I told you earlier - this is a limiting reagent question.

Assuming 100 mL of both solution - how much KOH will be left after reaction?

What will be the final volume?

What will be concentration of KOH left?

What will be the solution pH?

Why do I have to repeat it is a limiting reactant problem twice?

For the sake of generality you should do it not for 100 mL volumes, but for V volumes - V will cancel out in the end.