JEE Main 2025 — Ionic Equilibrium Question with Solution

To determine the mass of Mg(OH)₂ that needs to be dissolved to achieve a pH of 10.0, follow these steps:

Given:

pH = 10

Therefore, pOH = 14 - pH = 4

[OH⁻] = 10⁻⁴ M

First, calculate the number of moles of OH⁻ ions:

Number of moles of OH⁻ = 10⁻⁴

Since Mg(OH)₂ dissociates completely in water, from one mole of Mg(OH)₂, you get two moles of OH⁻:

Number of moles of Mg(OH)₂ = (10⁻⁴) / 2 = 5 × 10⁻⁵ moles

Next, calculate the mass of Mg(OH)₂:

Molar mass of Mg(OH)₂ = 58 g/mol

Convert the number of moles to mass:

Mass of Mg(OH)₂ = (5 × 10⁻⁵ moles) × (58 g/mol) = 2.9 × 10⁻³ g

Convert to milligrams: 2.9 × 10⁻³ g = 2.9 mg

Therefore, the calculated mass of Mg(OH)₂ required is approximately 2.9 mg.