Potassium trioxochlorate(V) (KClO₃) has a solubility of 1.5 mol dm⁻³ at 45°C. On cooling this solution to 20°C, the solubility becomes 0.5 mol dm⁻³. What mass of KClO₃ crystallized out from 1 dm³ of solution? [K = 39, Cl = 35.5, O = 16]

Correct answer: 122.50 g (Option D)

Explanation:

• Solubility at 45 °C = 1.5 mol dm⁻³.
• Solubility at 20 °C = 0.5 mol dm⁻³.
• From 1 dm³ of solution, moles that crystallize on cooling = \(1.5 - 0.5 = 1.0\) mol.
• Molar mass of KClO₃ = K (39) + Cl (35.5) + 3×O (3×16 = 48) = 122.5 g mol⁻¹.
• Mass crystallized = \(1.0 \text{ mol} \times 122.5 \text{ g mol}^{-1} = \mathbf{122.5\ \text{g}}\).

Why others are wrong: 12.25 g corresponds to 0.10 mol, 10.00 g to ~0.082 mol, and 1.00 g to ~0.008 mol—none match the 1.0 mol that must crystallize.