Question-​

Dear Student,

The solution is as follows:

Since we have been told to find the total Mg²⁺ concentration in the solution and not the one which is in equilibrium with MgF₂ , it will be the total concentration of Mg²⁺ in the solution, which is:

$$\begin{gathered} Total volume = 10 + 25 = 35 ml \\ Given: Initial \left[M{g}^{2+}\right]=0.25 M, Initial volume = 10 ml \\ Initial \left[F^{-}\right] = 0.2 M , initial volume = 35 ml \\ \left[M{g}^{2+}\right]=\frac{10 ml}{35 ml}\times 0.25 M=7.14 \times {10}^{-2} M \\ \left[F^{-}\right]=\frac{25 ml}{35 ml}x0.2 M=14 x {10}^{-2} M \end{gathered}$$
Thus, the correct answer is option (b).

Hope it helps.

Regards