Question

What mass of oxalic acid dihydrate , H2C2O4•2H2O, is required to prepare 220.0 ml of a 1.28 M solution of oxalic acid ?

Answer 1

How many mols do you need? That is M x L = 1.28M x 0.220L = ?
Then mol = g/molar mass. You know molar mass and you know mols, solve for grams.

Answer 2

M*L = 1.28M*0.220L = 0.282mols

molar mass = 126g

126g*0.282 = 35.5g