AlCl3(aq) is an ionic compound which will have the releasing of 3 Cl⁻ ions ions in water for every molecule of AlCl3 that dissolves.
AlCl3(s) --> Al+(aq) + 3 Cl⁻(aq)
[Cl⁻] = 0.28 mol AlCl3/1L × 3 mol Cl⁻ / 1 mol AlCl3 = 0.84 M
The answer to this question is [Cl⁻] = 0.84 M
