[TheDoctor123]

I have a few exercices in which I have to find the pH in some situations, but I cannot seem to find a way to do so, so if you guys could help it would be great. They are the following:

1 --> the pH of a solution that is the result of adding 50mL of a solution 0,100M HCl to 50mL of a solution containg 0,150M CH3CO2H and 0,20M CH3CO2Na.

2 --> the pH of a solution that contains 0,20 moles of CH3CO2Na in a litre of solution.

3 --> the pH of a solution 0,10M of Na2HPO4, justifying the approximations that have to be done.

[Dan]

You must show you have attempted the question, this is a Forum Rule.

[TheDoctor123]

Thing is, I really don't know how to do it. I do realize that if a I get to some point where I have H3O+ or OH- i can just -log(H30+) or -log(OH-), but I just don't know how to begin

[Vishal]

1) In this case find pH of buffer solution and to it use relationship of molarity and volume to get final solution.

2) In this case use the formula of pH.

3) Use the relationship of pH and pKa

[Borek]

Thing is, I really don't know how to do it. I do realize that if a I get to some point where I have H3O+ or OH- i can just -log(H30+) or -log(OH-), but I just don't know how to begin

Sorry, these are questions that require you to know much more than just pH definition - and they can't be answered in a short post. I suggest you start with any general chemistry book where the pH calculations are discussed, or with this set of lectures:

http://www.chembuddy.com/?left=pH-calculation&right=toc

[Borek]

1) In this case find pH of buffer solution and to it use relationship of molarity and volume to get final solution.

Partly true - it is a buffer related problem.

2) In this case use the formula of pH.

Which formula?

3) Use the relationship of pH and pKa

Which relationship?

[TheDoctor123]

So I was able to find the solution for the first and second exercices, but not for the last one. The only extra help I have is that Ka(H2PO4-)=6,3x10^-8. Any ideas?

[Borek]

The final one is an amphiprotic solution and needs two Ka values.