Practice Problem 3

Calculate the increase in the PCl3 and Cl2 concentrations that occur as the following reaction comes to equilibrium if the concentration of PCl5 decreases by 0.042 moles per liter.

PCl5(g) PCl3(g) + Cl2(g)


The decomposition of PCl5 has a 1:1:1 stoichiometry.

PCl5(g) PCl3(g) + Cl2(g)

For every mole of PCl5 that decomposes, we get one mole of PCl3 and one mole of Cl2. Thus, the change in the concentration of PCl5 that occurs as the reaction comes to equilibrium is equal to the change in the PCl3 and Cl2 concentrations. If 0.042 moles per liter of PCl5 is consumed as this reaction comes to equilibrium, 0.042 moles per liter of both PCl3 and Cl2 must be formed at the same time.