[Wheeler004]

I need some help getting the pH of some solutions:

-pH of a solution of 10.0 mL of 0.1 M NH4Cl plus 10.0 mL of 0.1 M NaOH
-pH of a solution of 10.0 mL of 0.1 M NH4Cl plus 15.0 mL of 0.1 M NaOH
-pH of a solution of 10.0 mL of 0.1 M NaC2H3O2 plus 10.0 mL of 0.1 M NaOH
-pH of a solution of 10.0 mL of 0.1 M NaC2H3O2 plus 15.0 mL of 0.1 M NaOH
-pH of 50 mL of solution that is 0.25 M in both H2PO41- and HPO42-.

For the life of me I can't figure out how to do these problems, so if someone could show me how I would appreciate it.  Thanks.

[Borek]

Write reaction equation whenever it looks like the reaction is possible. In most cases pH will be controlled by the excess of one reagent.

As for the last one - do you know Henderson-Hasselbalch equation?

[Wheeler004]

OK, thanks for the website.  After doing the equation for the last solution, I got 5.05.  It looks right to me, but it's being counted as wrong when I submit it.

[AWK]

For the last solution pH=pK₂of H₃PO₄= 7.2

[Borek]

After doing the equation for the last solution, I got 5.05.

Show how you get to 5.05.

[Wheeler004]

Okay, I got the last one using the H-H formula.  The first time I was just using pKa.  Any help on the first four?  How do I calculate the H3O+ concentration for those?

[Borek]

I hate to repeat myself.

Write reaction equation whenever it looks like the reaction is possible. In most cases pH will be controlled by the excess of one reagent.

[Wheeler004]

Yeah, I've tried that.  For the first one, there are 10ml of .1M acid and 10mL of .1M base.  Doesn't that mean that there is no excess of either?  I tried a pH of 7, but that's not the right answer.

[Borek]

Yeah, I've tried that.  For the first one, there are 10ml of .1M acid and 10mL of .1M base.  Doesn't that mean that there is no excess of either?  I tried a pH of 7, but that's not the right answer.

But you are close - just think it over. You started with witch a weak acid and you neutralized it with a stoichiometric amount of the strong base. What ions/molecules do you have in solution after that? Is there any that can change pH of the solution?