pH chemistry question

Can anyone please help with this chemistry buffer question part b

I got part a as 4.51 which i think is right, but how do i do part b

The original conc for ethanoic acid was 0.251 and for ethanoate was 0.140

I understand that CH3COOH -> CH3OO- + H+ is an equilibrium, so i thought if 0.015 moles of HCl was added, then I could answer the question by doing 0.140 - 0.015 and 0.251-0.015 and then plugging these values into the Ka equation
However, this does not get me the right value for pH

What step am I missing? is it something to do with the 1000cm^3?

So confused

Were you given the Ka constant for this earlier on in the question?

Yeah, its 1.74X10^-5

Well that makes things a lot more clear.

Write out the expression for Ka for the buffer solution.

There are two assumptions you will need to make for the concentration of the anion and the concentration of the acid.

Once that's figured out, you can find the concentration of H+ and do the appropriate calculation to find the pH.

I did that for question a! and got 4.51! But question B is the one I don't understand, it different because this is an equilibrium. Question B asks for the new pH after 0.015 mol of HCl are added to 1000cm^3 of the buffer solution