1.8 Chemical Equilibria, Le Chatelier's Principle & Kc

A compound, WX, will dissociate into an X^– ion and a W⁺ ion in solution. This is represented by the equation below.  

WX (aq) ⇌ X^– (aq) + W⁺ (aq)

2.0 dm³ of a 0.4 mol dm^-3 solution of WX was heated to a constant temperature until equilibrium was established. The equilibrium mixture is found to contain 0.1 mol of the W⁺ ion.  

What is the correct value of K_c?