Imagine that you are the quality control analyst supervising the synthesis of aspirin, HC9H7O4, a monoprotic acid. One tablet is required to contain 325 mg of aspirin. To analyze your product, you dissolve one tablet in 230. ml H2O, then titrate this solution with a 0.1000 M NaOH. You find that 18.87 mL of NaOH solution are required to reach the endpoint. How many mg of aspirin are actually in the tested sample tablet?

340.0mg of aspirin in the tested sample tablet

Explanation:

The titration of aspirin with NaOH obeys the following equation:

HC₉H₇O₄ + NaOH → C₉H₇O₄⁻ + Na⁺ + H₂O

As the complete titration spent 18.87 mL of 0.1000M NaOH, moles of NaOH are:

0.01887L × (0.1000mol / 1L) = 0.001887 moles of NaOH. As 1 mole of base reacts with 1 mole of aspirin, moles of aspirin are: 0.001887 moles of aspirin.

As molar mass of aspirin is:

C: 12.01g/mol × 9: 108.09g/mol

H: 1.01g/mol × 8: 8.08g/mol

O: 16g/mol ×4: 64g/mol

180.17g/mol. Mass of aspirin in the tested sample are:

0.001887 moles of aspirin × (180.17g / mol) = 0.3400g = 340.0mg of aspirin in the tested sample tablet