A solution contains F^- and CO₃^{2 -} both at 5.0×10^{- 5} M . Solid Ca(NO₃) ₂ is added slowly. At what concentration for Ca²⁺ ions does the second precipitate begin to form?
For CaF₂, K _sp = 1.7×10^{- 10}. For CaCO₃, K _sp = 4.7×10^{- 9}.

Question

Quizplus · Accepted Answer

The second precipitate will form when the concentration of Ca2+ ions reaches the solubility product of the solid. 
First, we need to determine which solid precipitates first. 
For CaF2: Ksp = [Ca2+][F-]^2 = (5.0×10^-5 M)(5.0×10^-5 M)^2 = 1.25×10^-14
For CaCO3: Ksp = [Ca2+][CO32-] = (5.0×10^-5 M)(4.7×10^-9 M) = 2.35×10^-13
The smaller Ksp value indicates that CaF2 will precipitate first. 
Next, we need to determine the concentration of Ca2+ ions when CaF2 begins to precipitate. 
Ksp = [Ca2+][F-]^2 
1.7×10^-10 = (x)(5.0×10^-5)^2 
x = 6.8×10^-12 M 
Therefore, the answer is B) 6.8×10^-12 M.

A Solution Contains F - and CO32 -

A Solution Contains F^- and CO₃^{2 -}