Question

1. A 100 mL sample is removed from a saturate aqueous solution of MgF₂. When the water is completely evaporated a 13 mg residue of MgF₂ (s) remains. What is K_sp for magnesium fluoride? Hint: Make sure you first write out the chemical equation; note the moles of each! (3.6 x 10^-8).

Answer 1

Mass of MgF2 = 0.013g
Volume of solution = 100 ml
Ksp of MgF2 = ?
solution :-
CaSO4 will dissociates in the following way:-
MgF2 ---------> Mg2+ 2F
thus,Ksp = [Ca2+] + [F] ^2
now to calculate the solubility of MgF2,as it can be expressed on moles/litre
first we calculate the no. of moles of MgF2 = Mass of MgF2
= 0.013
Molar mass of MgF2 =62.3018 g/mol
thus no. of moles = 2.08*10^-4
now solubility in no. of moles/litre = 2.08*10^-4 *
10000 = 2.08 moles/litre
100
thus CaSO4 on dissociation will give 2.08 mole of mG ions as well 0.018
moles of F2 ions
Ksp= [2.08]*[2.08] = 4.36