Question

A well is dug in a bed of rock containing fluorspar (CaF2). If the well contains 20000 L of water, what is the amount of F– in it ? Ksp = 4 × 10–11 (101/3 = 2.15)

• A. 4.3 mol
• B. 6.8 mol
• C. 8.6 mol
• D. 13.6 mol

Answer 1

Answer: (C)

8.6 mol

Answer 2

CaF2(s) $\rightleftharpoons$ Ca2+ + 2F–

4s3 = Ksp = 4 × 10–11 $\Rightarrow$ s = 2.15 × 10–4 M

So, amount of F– in 20000 lt of water = 2s × 20000 = 8.6 mol.