"An unknown amount of acid can often be determined by adding an excess of base and then "back-titrating" the excess. A 0.3471 g sample of a mixture of oxalic acid, which has two ionizable protons and benzoic, which has one, is treated with 100. mL of 0.1000 M NaOH. The excess NaOH is titrated with 20.0 mL of .2 M HCl. Find the mass % of benzoic acid."
Well I started by finding number of moles of OH
- actually used by finding total minus excess, which I got to be .01 moles -.004 moles = .006 moles of actual used OH
- in the neutralization reaction. After that I tried dividing by three since the mixture contains a total of 3 H
+ ions but benzoic acid only has one of the three, and I got .002 moles of benzoic acid times its molecular mass (122.12g/mol) and got .24424 g divided by the total (.3471g) multiplied by 100 and got 70.4%, however when I try to apply the same principles to oxalic acid, to try and check myself, I get a too large value and end up with .002 moles of oxalic acid times molecular mass (90.03g/mol) and get .18g divided by the total and multiplied by 100 which yields 51%... needless to say, these don't add up to 100% and I don't have any clue what to do now, any help would be appreciated