Calculate mass of potassium hydroxide (KOH) contained in 500 ml solution with pH = 11.5.

General Information:
Potassium hydroxide (KOH) is a strong base. It completely ionizes in solution to produce potassium ions (K⁺) and hydroxyl ions (OH^-). It has a molar mass (molecular weight) equal to 56.11 g/ mole.

Calculations:
pH = 14 + log [OH^-]
11.5 = 14 + log [OH^-]
-2.5 = log [OH^-]
[OH^-] = 10^-2.5 = 0.0032 mole/ liter.

[KOH] = [OH^-] = 0.0032 mole/ liter.

Volume of solution in liters = 500 ml × (0.001 liter/ ml) = 0.50 liter.
Number of moles = concentration × volume = (0.0032 mole/ liter) × (0.50 liter) = 0.0016 mole.
Mass of KOH = number of moles × molar mass = 0.0016 mole × (56.11 g/ mole) = 0.09 g.

Note: potassium hydroxide is a hygroscopic material (it picks up atmospheric moisture). Its moisture content can be greater than 15% in some cases. This moisture content should be accounted for when preparing solutions. Accurate concentration of prepared potassium hydroxide solutions can be obtained by titration with a strong acid (e.g. hydrochloric acid).

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