A 0.210-g sample of an acid (molar mass = 192 g/mol) is titrated with 30.5 mL of 0.108 M NaOH to a phenolphthalein endpoint. The formula of the acid is

Question

Quizplus · Accepted Answer

The acid reacts with NaOH in a 1:1 molar ratio, so moles of NaOH = moles of acid. Moles of NaOH = 0.108 M * 0.0305 L = 0.003294 moles. The molar mass of the acid is given as 192 g/mol, so moles of acid = 0.210 g / 192 g/mol = 0.001094 moles. The discrepancy in moles suggests the acid is polyprotic and fully reacts with 3 moles of NaOH per mole of acid, indicating the formula is H3A, which corresponds to choice A.

A 0210-G Sample of an Acid (Molar Mass = 192 G/mol)