Question

A chemist dissolves 249.mg of pure perchloric acid in enough water to make up 380.mL of solution. Calculate the pH of the solution. Be sure your answer has the correct number of significant digits.

Answer 1

pH = 2.18

Step-by-step explanation:

Perchloric acid (HClO₄) is a strong acid. This means that in an aqueous solution it completely dissociates into H⁺ and ClO₄⁻ species.

First we convert 249 mg HClO₄ into moles, using its molecular weight:

• 249 mg HClO₄ ÷ 100.46 mg/mmol = 2.49 mmol HClO₄

Because it is a strong acid, 2.49 mmol HClO₄ = 2.49 mmol H⁺

We calculate the molar concentration of H⁺:

• 2.49 mmol H⁺ / 380 mL = 6.52x10⁻³ M

Finally we calculate the pH of the solution:

• pH = -log[H⁺] = -log(6.52x10⁻³)

• pH = 2.18