OCR chemistry weak acid and base question

I done part a and got 2.73 but I'm not sure how to do part b

I done part a and got 2.73 but I'm not sure how to do part b

Sodium ethanoate contains CH3COO^- ions, which are the conjugate base of ethanoic acid, CH3COOH.

What has happened is you have formed a buffer solution containing 0.1 mol of CH3COO^- ions and 0.2 mol of CH3COOH and so now it is no longer true that [CH3COO^-] = [H^+].

Let [H^+] = x. Use the Ka expression again and solve for x. After finding x, what might be the logical thing to do with it?

use -log[H^+]

so im just rearranging the Ka= [H^+][A^-]/[HA] right?

also when using the formula it doesn't matter that I don't have the volume because it cancels out right?

Everything in your post is correct.

So have you managed to find the pH?

yes i got 4.45

thank you!

I (almost) agree with your answer. Have you looked at the third decimal place and rounded accordingly?

whoops... 4.46