An aqueous solution of a salt MX₂ at a certain temperature has a van't Hoff factor of 2. The degree of dissociation for this solution of the salt is:

MX₂ → M²⁺ + 2X⁻
α = (i - 1) / (n - 1)
where α is the degree of dissociation, i is the van't Hoff factor, and n is the number of ions produced from one formula unit of the electrolyte.
Given i = 2 and n = 3 (1 M²⁺ ion and 2 X⁻ ions),
α = (2 - 1) / (3 - 1) = 1 / 2 = 0.5