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)

Answer

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.