How to Calculate the pH of a Buffer Solution

[kirsten_2009]

Homework Statement

Hi, I really don't get how to calculate the pH of a buffer solution...I mean I know that the pH of a solution is the negative log of the H3O+ concentration but I don't know how to get to that part...here's the problem...and I would appreciate if someone could actually explain it to me rather than just posting the answer. Thanks so much in advance!

Q:// In a beaker mix 20 mL of 0.1 M Acetic Acid and 25 mL of 0.1 M Sodium Acetate. Calculate the pH of this solution.

Q2:// Also, then add 5 mL of 0.1 M NaOH to this buffer and re-calculate the pH.

Homework Equations

I believe it would be pH=-log[H3O+]

The Attempt at a Solution

I don't have much of an idea...help please :)

 

[Bohrok]

For a buffer solution, you'll usually want to use the Henderson-Hasselbalch equation:
pH = pK_a + log([salt]/[weak acid])

where pK_a = -log(K_a), [weak acid] is the concentration of the weak acid and [salt] is the concentration of the conjugate base, or salt of the weak acid.

Since you have relatively dilute solutions of the acetic acid and sodium acetate, you can add their volumes to get the total volume of solution. Then you can find the molarities of the acid and salt in the solution and plug them into the equation to get pH.