[idankor]

Sorry about my english:

need to calculate the pH of 2 solutions mixed together:

120 ml H3PO4 0.15 Normal
40 ml KOH 0.4 Molar

Ka1 = 7.5 x 10^-3
Ka2 = 6.2 x 10^-8
Ka3 = 4.8 x 10^-13

What I did, but apparently is incorrect:

0.15 N ---> 0.05 M ---> 0.006 moles of the acid
0.016 moles of the base - 0.006 = 0.01
and I found the [OH-] and then the [H3O+]. But - my answer is slightly different then the one in the book, and secondly I did not use the Ka's.

So, any suggestions of the steps I should take ?
What reactions are taking place here ?

Thank you in advance.

[Borek]

What reactions are taking place here ?

You tell us what hqappens when you mix acid and base.

Have you heard about buffer solutions?

[idankor]

Absolutely yes, but still can't figure this one out. I know the Henderson–Hasselbalch equation, But I really can't figure out what's going on here or in other words: how to approach to this problem. Once someone will help me out I'm sure I'd do better alone next times. Thanks!

[blightcutter]

From the Ka values you can construct a titration curve to find the pH as the reaction reaches its end-point. Do you see a strong acid? What reactions will happen first?

[idankor]

Nope, I see a weak acid and a strong base. I don't know what reactions take place and which one happens first. I am too confuse so I won't try to guess. I want to start from scratch. What should I do first ? and how ?

If someone can give some guidance lines that would be great, because I'm not so sure in myself.
Thanks again, you're great guys.

[Rabn]

The first thing you need to do is write out all of the dissociation reactions. Heck I'll even give you the second thing you need to; write out all pertinent equilibrium equations. 

[idankor]

Yeah, well, I asked what are they, that was my problem, didn't know them. But I'm happy to say I solved the problem ! So thanks to all anyways ! 

[Borek]

Neutralization was a key word here.