Question

Calculate the concentration of f- in a solution saturated with mgf2 and srf2. Given ksp of mgf2 is 9.5*10-9 and ksp of srf2 is 4*10-9

Answer 1

Answer: Total  Concentration of fluorine ions $[F^-]=1.59\times 10^{-4}mol./L$

Explanation:

1) $MgF_2\rightleftharpoons Mg^{2+}+2F^-$

         S                2S

$K_{sp}$  of $MgF_2$ = $9.5\times 10^{-9}=[S][2S]^2=4S^2$

$S=4.87\times 10^{-5} mol/L$

Concentration of fluorine ions in saturated solution of $MgF_2$

$[F^-]_{MgF_2}=2S=2\times 4.87\times 10^{-5} mol/L=9.74\times 10^{-5}mol/L$...(1)

2) $SrF_2\rightleftharpoons Sr^{2+}+2F^-$

             S'                2S'

$K_{sp}$ of $SrF_2$ = $4\times 10^{-9}=[S'][2S']^2=4S'^2$

$S'=3.1\times 10^{-5}mol/L$

Concentration of fluorine ions in saturated solution of $SrF_2$

$[F^-]_{SrF_2}=2S=2\times 3.1\times 10^{-5} mol/L=6.2\times 10^{-5}mol/L$...(2)

Total  Concentration of fluorine ions $[F^-]$

$=[F^-]_{MgF_2}+[F^-]_{SrF_2}=9.74\times 10^{-5}mol/L+6.2\times 10^{-5}mol/L=15.94\times 10^{-5}mol/L=1.59\times 10^{-4}mol./L$

Answer 2

Answer: The concentration of $F^-$ is $4.668\times 10^{-3}M$

Explanation: To calculate the concentration of the$F^-$ ion, we need to write the equations for the ionization of these salts.

1. Ionization of $MgF_2$

         $MgF_2\rightleftharpoons Mg^{2+}+2F^-$

                    s              2s

$K_{sp}$ of $MgF_2=9.5\times 10^{-9}=[s][2s]^2=4s^3$

$s=1.334\times 10^{-3}M$

Concentration of fluoride ions in saturated solution of $MgF_2=2s=(2\times 1.334)\times 10^{-3}M=2.668\times 10^{-3}M$

2. Ionization of $SrF_2$

        $SrF_2\rightleftharpoons Sr^{2+}+2F^-$

               s'                2s'

$K_{sp}\text{ of }SrF_2=4\times 10^{-9}=[s'][2s']^2=4s'^3$

$s'=1\times 10^{-3}M$

Concentration of fluoride ions in saturated solution of $SrF_2=2s'=(2\times 1)\times 10^{-3}M=2\times 10^{-3}M$

Total concentration of fluoride ions = 2s + 2s' = $(2.668+2)\times 10^{-3}M=4.668\times 10^{-3}M$