A $100 mL$ solution contains oxalic acid ( $H X_{2} C X_{2} O X_{4}$ ) and sodium oxalate ( $Na X_{2} C X_{2} O X_{4}$ ). A $20.0 mL$ portion needs $24.0 mL$ of $0.100 M$ $NaOH$ with phenolphthalein (only the free acid is neutralised, both protons). Another $20.0 mL$ portion needs $30.0 mL$ of $0.0400 M$ $KMnO X_{4}$ in hot acid. Find the molarity of $Na X_{2} C X_{2} O X_{4}$ in the solution (three decimals).

Numerical answer type, answer in M, tolerance 0.001.

Answer: 0.09 M

H2C2O4 = (0.0240 x 0.100)/2 = 1.20e-03 mol per 20 mL. Total oxalate = 2.5 x (0.0300 x 0.0400) = 3.00e-03 mol. Na2C2O4 = 3.00e-03 - 1.20e-03 = 1.80e-03 mol, so molarity = 1.80e-03/0.0200 = 0.090 M.

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Answer: 0.09 M

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