The percentage dissociation of a salt (MX₃) solution at given temperature (van't Hoff factor i=2 ) is ___________ %(Nearest integer)

Solution

Chemistry ma’am · 4 steps

Count the ions MX₃ gives first, then let i = 1 + (n-1)α do the rest.

1. Count the ions. MX₃ → M³⁺ + 3X⁻, so one formula unit gives n = 1 + 3 = 4 ions.
2. Particles after dissociation. Start with 1 mol of salt and let a fraction α dissociate. You have 1 - α mol undissociated and 4α mol of ions, so the total is 1 + 3α. That total is the van't Hoff factor.

   i = 1 + (n-1)α ⇒ 2 = 1 + 3α

3. Solve for alpha. 3α = 1, so α = 1/3 = 0.333, which is 33.3%.
4. So. The salt is about 33.3% dissociated, which rounds to 33.

The common slip: 50. Taking n = 3 instead of 4 (counting only the three X⁻ ions) gives α = 1/2, that is 50, not 33.

The idea to keep. For dissociation, α = (i-1)/(n-1), where n counts every ion, cation and anions together. Write the dissociation equation first so you do not miscount n.

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