Question

A well is dug in a bed of rock containing fluorspar $\left(Ca{F}_2\right)$. If the well contains 20000 L of water, what is the amount of $F^{-}$ in it? ($K_{sp}=4\times {10}^{-11}$)

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

Answer 1

The correct option is C 8.6 mol

The dissociation equation is$Ca{F}_2\left(s\right)\rightleftharpoons C{a}^{2+}\left(aq\right)+2F^{-}\left(aq\right)t=0100ateqb1-ss2s$

$K_{sp}=\left[C{a}^{2+}\right]+[F^{-}{]}^2$
$K_{sp}=s\times (2s{)}^2=4s^3$
$K_{sp}=4s^3=4\times {10}^{-11}$
$\Rightarrow s=2.15\times {10}^{-4}M$
So, amount of $F^{-}$ in 20000 L of water $=2s\times 20000=2\times 2.15\times {10}^{-4}\times 20000=8.6mol$