A 0.500 g sample of an unknown monoprotic solid acid (HA) is dissolved in water and…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A 0.500 g sample of an unknown monoprotic solid acid (HA) is dissolved in water and titrated with 0.1000 M NaOH at 25 °C. A pH-meter indicates that stoichiometric neutralization occurs after 31.44 mL of NaOH have been added. In a separate trial that uses only the indicator phenolphthalein, the student observes the endpoint after 31.94 mL of NaOH have been added.
(a) Derive an algebraic expression for the molar mass M of the acid in terms of the sample mass m, the molarity C of the NaOH, and the true equivalence volume Veq.
(b) Using the true equivalence volume, calculate the actual molar mass of the acid.
(c) Using the indicator endpoint volume, calculate the molar mass the student would obtain.
(d) Determine the percent deviation of the student’s value from the true molar mass and state whether the acid would appear to have a larger or smaller molar mass than it actually does. Explain the chemical reason for this direction of error.