Question #bc52a

To find the percent dissociation of phosphorus pentachloride, you need to divide the concentration that dissociated to produce phosphorus trichloride and chlorine gas by the initial concentration of the reactant and multiply the result by #100%#.

You know that the initial concentration of the reactant is equal to #"2 M"#. At equilibrium, the concentration of the reactant is said to be equal to #"1.5 M"#.

This implies that the concentration of phosphorus pentachloride decreased by

#"2 M " - " 1.5 M = 0.5 M"#

In other words, #"0.5 M"# of phosphorus pentachloride dissociated to produce, according to the balanced chemical equation, #"0.5 M"# of phosphorus trichloride and #"0.5 M"# of chlorine gas.

You can thus say that the percent dissociation is equal to

#"% dissociation" = (0.5 color(red)(cancel(color(black)("M"))))/(2color(red)(cancel(color(black)("M")))) * 100% = color(darkgreen)(ul(color(black)(25%)))#

SIDE NOTE The units given to you for the concentration of the reactant, i.e. #"g mol L"^(-1)#, are equivalent to #"mol L"^(-1)#, which, as you know, are used for the molarity of the reactant.