The Student Room Group
ok, this is a typical buffers question

1. Calculate the moles of acid left after the reaction and then the concentration of the acid
2. Calculate the moles of salt produced by the reaction and then the concentration of the salt in the solution
3. Apply the buffer law expression derived from Ka= [H+][A-]/[HA]

for more help on buffers see http://www.ibchem.com/IB/ibnotes/full/aab_htm/9.4.htm