Chemistry

Problem

Fluoride ion is added slowly to a well-stirred mixture that is 0.010 M in both

Ca^{2+}

and

Mg^{2+}

. The solubility products are

K_{sp}(CaF_2)=3.9\times10^{-11}

and

K_{sp}(MgF_2)=6.4\times10^{-9}

. Which of the following best explains which fluoride precipitates first? a) Both fluorides precipitate simultaneously because the solutions contain equal concentrations of

Ca^{2+}

and

Mg^{2+}

. b)

MgF_2

precipitates first because its larger

K_{sp}

means the saturation limit is reached at a lower

[F^-]

than for

CaF_2

. c)

CaF_2

precipitates first because its smaller

K_{sp}

means the ion product reaches saturation at a lower

[F^-]

than for

MgF_2

. d) Neither fluoride precipitates until

[F^-]

exceeds

6.4\times10^{-9}\,\text{M}

, the larger of the two

K_{sp}

values.

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