Correct answer: 12.16 × 10⁻³
To calculate the milli-equivalents of washing soda required, first determine the milli-equivalents of Mg²⁺ in 1 litre of water. Mg²⁺ has a molar mass of 24 g/mol and a valency of 2. The milli-equivalents of Mg²⁺ = (12 mg / 24) × 2 = 1 milli-equivalent. Since 1 milli-equivalent of washing soda is required to remove 1 milli-equivalent of Mg²⁺, the answer is 12.16 × 10⁻³.