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
You know that the initial concentration of the reactant is equal to
This implies that the concentration of phosphorus pentachloride decreased by
#"2 M " - " 1.5 M = 0.5 M"#
In other words,
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.