How many moles of benzoic acid, a monoprotic acid with K_a = 6.4 × 10^-5, must be dissolved in 500. mL of H₂O to produce a solution with pH = 2.50?

Question

Quizplus · Accepted Answer

To find the moles of benzoic acid needed, use the formula for pH and the expression for the dissociation of a weak acid. The pH of 2.50 corresponds to [H⁺] = 3.16 × 10⁻³ M. Using the formula Ka = [H⁺]² / [HA], solve for [HA] (initial concentration of benzoic acid). This gives [HA] = 7.8 × 10⁻² M. Since the volume is 0.5 L, the moles of benzoic acid needed is 7.8 × 10⁻² M × 0.5 L = 3.9 × 10⁻² moles.

How Many Moles of Benzoic Acid, a Monoprotic Acid with Ka

How Many Moles of Benzoic Acid, a Monoprotic Acid with K_a