The concentrations of the products at equilibrium are [pcl3] = 0.180 m and [cl2] = 0.250 m. what is the concentration of the reactant, pcl5, at equilibrium?

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