A reaction CaF2 ⇌  Ca+2  +  2F-  is at equilibrium. If the concentration of Ca2+  is increased four times, what will be the change in F-  concentration as compared to the initial concentration of F-  ?

The dissociation constant K of the reaction = Ca2+ F-2 CaF2  When the concentration of Ca2+  increases and becomes four times, the concentration of F-  must decrease by the same value. This is necessary in order to maintain K as constant. Since the F-  concentration is raised to the power 2 in the expression, the concentration of fluoride ion must decrease by 1/2.