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11/10/2021 10:39 am
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If Ksp of CaF2 at 25°C is 1.7 × 10-10, the combination amongst the following which gives a precipitate of CaF2 is
(a) 1 × 10-2 M Ca2+ and 1 × 10-3 M F-
(b) 1 × 10-4 M Ca2+ and 1 × 10-4 M F-
(c) 1 × 10-2 M Ca2+ and 1 × 10-5 M F-
(d) 1 × 10-3 M Ca2+ and 1 × 10-5 M F-
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11/10/2021 10:51 am
Correct answer: (a) 1 × 10-2 M Ca2+ and 1 × 10-3 M F-
Explanation:
When ionic product i.e. the product of the concentration of ions in the solution exceeds the value of solubility product, formation of precipitate occurs.
CaF2 ⇌ Ca2+ + 2F-
Ionic product = [Ca2+][F-]2
when, [Ca2+] = 1 x 10-2 M
[F-]2 = (1 x 10-3)2M
= 1 x 10-6 M
∴ [Ca2+][F-]2 = (1 x 10-2)(1 x 10-6)
= 1 x 10-8
In this case,
Ionic product (1 x 10-8) > solubility product (1.7 x 10-10)