The observed and normal molar masses of compound MX₂ are 65.6 and 164 respectively. The percent degree of ionisation of MX₂ is __________%. (Nearest integer)

Solution

Chemistry ma’am · 3 steps

The observed molar mass is too small, so the solute has split. Get i from the ratio, then use n = 3.

1. Find i. Colligative properties depend on the number of particles. When a solute dissociates, the molar mass you observe drops by the same factor, so i = (normal molar mass)/(observed molar mass) = 164/65.6 = 2.5.
2. Count the ions. MX₂ → M²⁺ + 2X⁻, so one formula unit gives n = 3 ions. If a fraction α dissociates, the particles per formula unit become 1 - α + 3α = 1 + 2α.

   α = (i - 1)/(n - 1) = (2.5 - 1)/(3 - 1) = 1.5/2 = 0.75

3. So. The degree of ionisation is 0.75, that is 75%.

The common slip: 50. Dividing i - 1 by n = 3 instead of n - 1 = 2 gives 50, not 75.

The idea to keep. For dissociation, i is normal over observed molar mass and α = (i-1)/(n-1). Always count n from the balanced ionisation, including every ion.

Answer from the NTA final answer key. Solution by PrepWiser.