Question

Calculate the number of moles of MgF2 that dissolved.

Answer 1

Moles of MgF₂ : 1.2 x 10⁻⁴

Further explanation

Maybe the complete question is like this

A student prepares 100. mL of a saturated solution of MgF2 by adding 0.50 g of solid MgF2 to 100. mL  of distilled water at 25°C and stirring until no more solid dissolves. (Assume that the volume of the  undissolved MgF2 is negligibly small.) The saturated solution is analyzed, and it is determined that [F−] in  the solution is 2.4 × 10−3 M.

The dissociation reaction of MgF₂

MgF₂(s)⇒ Mg²⁺(aq)+2F⁻(aq)

mol ratio MgF₂ : F⁻ = 1 : 2

mol of F⁻ in 100 ml solution :

$\tt mol=M\times V\\\\mol=2.4\times 10^{-3}\times 0.1=2.4\times 10^{-4}$

mol MgF₂ :

$\tt \dfrac{1}{2}\times 2.4\times 10^{-4}= 1.2\times 10^{-4}$