Fluoride ion is added slowly to a well-stirred mixture that is 0.010 M in both and
CHEMISTRY · Single correct · AP · Chemistry · Equilibrium · Equilibrium
Problem
Fluoride ion is added slowly to a well-stirred mixture that is 0.010 M in both Ca2+ and Mg2+. The solubility products are Ksp(CaF2)=3.9×10−11 and Ksp(MgF2)=6.4×10−9. Which of the following best explains which fluoride precipitates first?
(a) Both fluorides precipitate simultaneously because the solutions contain equal concentrations of Ca2+ and Mg2+.
(b) MgF2 precipitates first because its larger Ksp means the saturation limit is reached at a lower [F−] than for CaF2.
(c) CaF2 precipitates first because its smaller Ksp means the ion product reaches saturation at a lower [F−] than for MgF2.
(d) Neither fluoride precipitates until [F−] exceeds 6.4×10−9M, the larger of the two Ksp values.