I'm thinking the Mg reacts with the propanoic acid to produce propanoate. So you have 1mole each of acid and propanoate to start, as it is 1moldm-3 and you have 1dm3 vol.
Work out moles of Mg. Moles of acid decreases by number of moles of Mg, and moles of propanoate increase by moles of Mg. You now have new moles of each present, which is the same ratio as concentration. Rearrange Ka expression to give H+. Sub in your values to work out [H+] and then convert to pH.