A mixture of oxalic acid ( $H X_{2} C X_{2} O X_{4}$ ) and sodium oxalate is dissolved in water. A $25.0 mL$ portion needs $13.2 mL$ of $0.1000 M$ $NaOH$ to the phenolphthalein end point (sodium oxalate does not react with the base). Another $25.0 mL$ portion, acidified and warmed, needs $27.5 mL$ of $0.0200 M$ $KMnO X_{4}$ . Find the concentration of sodium oxalate in the mixture in $mmol L^{- 1}$ , to the nearest integer.

Numerical answer type, answer in mmol/L, tolerance 0.5.

Answer: 29 mmol/L

H X_{2} C X_{2} O X_{4}

= 13.2 \times 0.1000/2 = 0.660

mmol per

25.0 mL

. Total oxalate from permanganate

= 27.5 \times 0.0200 \times 5/2 = 1.375

mmol. Sodium oxalate

= 1.375 - 0.660 = 0.715

mmol, so concentration

= 0.715/25.0 \times 1000 = 28.6 mmol L^{- 1}