# Acids, Bases, and Buffers - Calculate Ka and pH?

1. Oct 22, 2013

### emilyelisabeth

1. The problem statement, all variables and given/known data
If a weak acid, HA, is 3% dissociated in a 0.25 M solution, calculate the Ka and the pH of the solution.

2. Relevant equations
pH = (pKa - log[HA])/2

3. The attempt at a solution
Honestly, I'm not even sure where to start. I don't have the pH or the pKa - how am I suppose to solve for either one?

I'm really struggling with this worksheet, so I'm sure this won't be my last question.

2. Oct 22, 2013

### emilyelisabeth

I think I may have figured it out...

3. Oct 22, 2013

### epenguin

Your equation is OK but is not helping you.

I recommend going back to basics, otherwise you will always be stuck with these problems.

To solve these problems from first principles there are in general 3 equations to be written out:

A mass balance equation;

an electroneutrality equation ;

an equilibrium equation (definition of the relevant equilibrium constant).

(Then there is often a simplifying approximation to be written, but this is optional in this case).

Do this for all your problems, do not rely on equations like your quoted one unless you are very sure of their rationale.

My helpful hint in this case is just do the first two. Then you should see you can obtain [H+] from the information given. After that you will be able to get Ka, pH and pKa.