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J. W. Hill, R. H. Petrucci, T. W. McCreary, & S. S. Perry General Chemistry 4th edition

Problem 16.51

In hard water, [Ca2+] is about 2.0×10–3 M. Water is fluoridated with 1.0 g F per 1.0×103 L of water. Will CaF2(s) precipitate from hard water upon fluoridation?




Answer:

Find the reaction quotient for CaF2 and compare it to Ksp.

CaF2(s)Ca2+(aq) + 2 F(aq)

Q = [Ca2+]init[F]2init

[Ca2+]init = 2.0×10–3 M

[F]init = (1.0 g)/(19.0 g/mol)/( 1.0×103 L) = 5.3×10–5 M

so Q = [2.0×10–3][ 5.3×10–5]2 = 5.6×10–12

Ksp = 5.3×10–9

Since Ksp > Q, there are not enough ions to satisfy the equilibrium condition so no precipitation occurs.