A 20.00 mL oxalic acid sample needs 23.24 mL of 0.09113 M KMnO4, reacting as 2MnO4- with 5H2C2O4. What is the acid molarity?

1. Work in millimoles, since molarity also equals millimoles of solute per millilitre of solution. 2. Millimoles of MnO4- used equal 23.24 mL times 0.09113 mmol/mL, which is 2.11786 mmol. 3. The equation shows 5 mol of oxalic acid for every 2 mol of permanganate. 4. So millimoles of oxalic acid equal 2.11786 times 5 divided by 2, which is 5.29464 mmol. 5. Dividing by the 20.00 mL sample volume gives 0.26473 mmol/mL. 6. Reported to four significant figures the answer is 0.2648 M. 7. Skipping the 5 to 2 factor gives 0.1059 M, and inverting it gives 0.04235 M, the two usual traps. _Source: OpenStax Chemistry (CC BY 4.0), Ch 4 "Stoichiometry of Chemical Reactions", section 4.5 Quantitative Chemical Analysis_