We have Kc = 4.2 x 10^-2 (given but missing in the question) and When the balanced equation for this reaction is: PCl5(g) ↔ PCl3(g) + Cl2(g) so, according to the Kc formula: Kc = the concentration of products / the concentration of the reactants so, to get the concentration of the reactants in equilibrium, the concentration of the products / the concentration of the reactants should equal the Kc value which is given in the question (missing in your question). So by substitution in Kc formula: Kc = [PCl3]*[Cl2] / [PCl5] 4.2 x 10^-2 = 0.18 * 0.25 /[PCl5] ∴[PCl5] = 0.18*0.25 / 4.2x10^-2 = 1.07 So the concentration of the reactants in equilibrim = 1.07
