What is pH of a 400 ml aqueous solution that contains 1.0 g potassium hydroxide (KOH).

Number of moles = mass / molar mass

Molar mass (molecular weight) of KOH = 56.11 g/ mole

Number of moles = 1.0 g/ (56.11 g/ mole) = 0.018 mole

Molar concentration = number of moles/ volume of solution in liters

Volume of solution in liters = 400 ml/ 1000 = 0.4 L

Molar concentration = 0.018 mole/ (0.4 L) = 0.045 M

pOH = -log [OH^-]

[OH^-] = 0.045 M

pOH = - log 0.045 = 1.35

pH + pOH =14

pH = 14 - pOH = 14 - 1.35 = 12.65