Ionic EquilibriumHard

Question

When pure water is saturated with CaCO3 and CaC2O4, the concentration of calcium ion in the solution under equilibrium is 8 × 10-5 M. If the ratio of the solubility product of CaCO3 to that of CaC2O4 is 3, what is the solubility product of CaCO3 in pure water?

Options

A.4.80 × 10-8
B.9.60 × 10-9
C.9.60 × 10-8
D.4.80  × 10-9

Solution

CaCO3 ⇋  Ca+2 + CO3-2   CaC2O4 ⇋  Ca+2 + C2O4-2   
                      x            x                                 y            y
                  (x+y)          x                             (x+y)         y
= 3  ⇒  x = 3y
(x + y)  = 8 × 10-5 
(3y + y) = 8 × 10-5  ⇒   y = 2 × 10-5  x = 6 × 10-5
KSP (CaCO3)    =    [Ca+2] [CO3-2]
                           =    8 × 10-5 × 6 × 10-5
                           =    48 × 10-10
KSP (CaCO3)    =    4.8 × 10-9

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