Question

If 1.2 ·10^-14 moles of magnesium hydroxide, Mg(OH)2, are dissolved in 1.0 L of aqueous solution, what are [OH-] and [H30+]?

Answer 1

1. [OH⁻] = 2.4 x 10⁻¹⁴ M.

2. [H₃O⁺] = 4.167 x 10⁻¹ M.

Step-by-step explanation:

Firstly, we need to calculate the molarity of Mg(OH)₂:

• Molarity (M) is the no. of moles of solute that dissolved in 1.0 L of the solution.

M = (no. of moles of solute)/(V of the solution (L).

∴ M of MgCl₂ = (no. of moles of solute)/(V of the solution (L) = (1.2 x 10⁻¹⁴ mol)/(1.0 L) = 1.2 x 10⁻¹⁴ M.

1. [OH⁻]:

• Mg(OH)₂ is dissociated in the solution according to the equation:

Mg(OH)₂ → Mg²⁺ + 2OH⁻.

• It is clear that every 1.0 mol of Mg(OH)₂ produces 2.0 moles of (OH⁻).

∴ [OH⁻] = 2(1.2 x 10⁻¹⁴) = 2.4 x 10⁻¹⁴ M.

2. [H₃O⁺]:

∵ [H₃O⁺][OH⁻] = 10⁻¹⁴.

∴ [H₃O⁺] = 10⁻¹⁴/[OH⁻] = 10⁻¹⁴/(2.4 x 10⁻¹⁴ M) = 4.167 x 10⁻¹ M.