Calculate the molarity of the student's sodium hydroxide solution. Be sure your answer has the correct number of significant digits.

Question

A chemistry student needs to standardize a fresh solution of sodium hydroxide. He carefully weighs out 40.mg

 of oxalic acid H2C2O4

, a diprotic acid that can be purchased inexpensively in high purity, and dissolves it in 250.mL

 of distilled water. The student then titrates the oxalic acid solution with his sodium hydroxide solution. When the titration reaches the equivalence point, the student finds he has used 35.6mL

 of sodium hydroxide solution.

Calculate the molarity of the student's sodium hydroxide solution. Be sure your answer has the correct number of significant digits.

Answer 1

H2C2O4 + 2NaOH ==> Na2C2O4 + 2H2O ... balanced equation for the reaction taking place

Molar mass oxalic acid = 90.03 g / mol

moles H2C2O4 present = 40 mg x 1 g / 1000 mg x 1 mol / 90.03 g = 4.44x10-4 moles

moles NaOH required = 4.44x10-4 mols H2C2O4 x 2 mols NaOH / mol H2C2O4 = 8.88x10-4 mols

[NaOH] = 8.88x10-4 mols / 35.6 ml x 1000 ml / L = 0.0249 M = 0.025 M (2 sig. figs.)