What mass of oxalic acid needs to be taken so that 20 ml of 0.1 M sodium hydroxide solution are consumed for its titration?

Given:
V solution (NaOH) = 20 ml = 0.02 l
Cm (NaOH) = 0.1 M

To find:
m (HOOC-COOH) -?

Decision:
1) HOOCCOOH + 2NaOH => NaOOCCOONa + 2H2O;
2) n (NaOH) = Cm * V solution = 0.1 * 0.02 = 0.002 mol;
3) n (HOOCCOOH) = n (NaOH) / 2 = 0.002 / 2 = 0.001 mol;
4) m (HOOCCOOH) = n * M = 0.001 * 90 = 0.09 g.

Answer: The mass of HOOCCOOH is 0.09 g.