year 13 alevel chemistry

I need help with this question:
Calculate the mass of sodium ethanoate required to dissolve in 50 cm3 of 0.100 mol dm-3 ethanoic acid to make a pH 5.20 buffer (Ka for ethanoic acid = 1.76x10-5)

I found the moles of ethanoic acid which is 5x10^-3
I found [H+] by doing 10^-5.20 which is 6.31 x 10^-3
then I rearranged the Ka equation to find [A-] and I got 1.4 x 10^-2
I don't know where to go from here! I have the answer but I don't know how to get to there because everything I have tried has given me a different answer to it

Reply 1

BankaiGintoki

Original post by _unknown_123

I need help with this question:

I found the moles of ethanoic acid which is 5x10^-3
I found [H+] by doing 10^-5.20 which is 6.31 x 10^-3
then I rearranged the Ka equation to find [A-] and I got 1.4 x 10^-2
I don't know where to go from here! I have the answer but I don't know how to get to there because everything I have tried has given me a different answer to it