a) The solubility of magnesium fluoride , MgF 2 , in water is 1.5E-2 g/L. What i

Question

a)

The solubility of magnesium fluoride, MgF2, in water is 1.5E-2 g/L. What is the solubility (in grams per liter) of magnesium fluoride in 0.11 M of sodium fluoride, NaF?

= _____g/L

b)

Calculate the solubility of strontium sulfate, SrSO4, in 8.00E-2M of sodium sulfate, Na2SO4. Solve the equation exactly. Ksp(SrSO4) = 2.5E-7

= _____g/L

c)

Magnesium sulfate, MgSO4, is added to 81.0 mL of 3.90E-2M sodium hydroxide, NaOH, until precipitate just forms. How many grams of magnesium sulfate were added? Assume that the volume of the solution is not changed significantly by the addition of magnesium sulfate.
Ksp Mg(OH)2 = 1.8E-11

= _____g

Explanation / Answer

a) The solubility of magnesium fluoride, MgF2, in water is 1.5E-2 g/L. What is the solubility (in grams per liter) of magnesium fluoride in 0.11 M of sodium fluoride, NaF?

Solution

                 MgF2 ---> Mg+2   + 2F-

solubility of MgF2 = 1.5 X 10^-2 g / L = 1.5 X 10^-2 / molecular wt moles / L = 1.5 X 10^-2 / 62.3 = 2.41 X 10^-4 M

This will give 2X 2.41 X 10^-4 M of F- = 4.82 X 10^-4 Molar

so Ksp of MgF2 = [Mg2+][2F-]^2 = 2.41 X 10^-4 X [4.82 X 10^-4 ]^2 = 5.59 X10^-11

Cocentration of NaF = 0.11 M

MgF2. --> Mg2+.....F-
Iniital       .0          0.11
Change     +x        +2x
equilbirium   x       0.11+2x

Ksp MgF2 = 5.59 X10^-11 = [x][0.11 + 2x]^2

we can ignore 2x as Ksp is very low
5.59 X10^-11 = 0.0121 x
x = 4.61 X 10^-9 Molar

cocnentration in g / L = 4.61 X 10^-9 X 62.3 = 2.87 X 10^-7 g/L

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