**Practice Problem 3**

Calculate the increase in the PCl_{3} and Cl_{2}
concentrations that occur as the following reaction comes to
equilibrium if the concentration of PCl_{5} decreases by
0.042 moles per liter.

PCl_{5}(*g*) PCl_{3}(*g*)
+ Cl_{2}(*g*)

**Answer**

The decomposition of PCl_{5} has a 1:1:1
stoichiometry.

PCl_{5}(*g*) PCl_{3}(*g*)
+ Cl_{2}(*g*)

For every mole of PCl_{5} that decomposes, we get one
mole of PCl_{3} and one mole of Cl_{2}. Thus, the
change in the concentration of PCl_{5} that occurs as the
reaction comes to equilibrium is equal to the change in the PCl_{3}
and Cl_{2} concentrations. If 0.042 moles per liter of
PCl_{5} is consumed as this reaction comes to
equilibrium, 0.042 moles per liter of both PCl_{3} and Cl_{2}
must be formed at the same time.