More on the Common Ion Effect During discussion on the common ion effect, the following question was raised: When two saturated solutions of ionic compounds AC and BC are mixed with C- as the common ion, will a precipitate form? While it may seem that the concentration of C-should increase beyond the saturation point of the less soluble compound, it must be noted that when the solutions are mixed, the total volume increases. Therefore, the concentration of C- may not increase sufficiently for precipitation to occur. The formation of precipitate will require a specific difference in the solubility product constants of the two compounds. The following is a quantitative determination of that difference. One liter of saturated solution AC is added to 1I of saturated solution BC. The magnitudes of the charges onall the ions are equal. The soluhility product constants of AC and BC are K1 and Kp, respectively. If AC is the less soluble compound, K1 will be less than Kz. ACu =Ac(aq.) C-(aq.) Solution ( I ) K I = [Atl[C-I
+
Solution (2) In solution (11, thus
BC(.)
+ B+(aq.) + C - ( a d [A+] = [C-] [A+] =
Kz = [Bf][C-]
K , = [A+I2 = [C--]2 and [C-] =
a
and in the two-liter mixture
In solution (2) In the two-liter mixture
For precipitate to form Substitution yields
Multiplying through
Hence Subtracting K I Therefore, Since solubility product constants are always greater than zero, dividing by K1 yields
K2 > 9K1 If Kz is more than nine times ss great as K1 precipitation will occur. This can he predicted simply by comparing the constants without further calculation. Walter Pape Loyola Academy Willmlte. IL 60091
Volume 58
Number 12
December 1981
1019
