Calculating the solubility product constant, given the solubility?

Question

On the textbook it says, I've been given magnesium fluoride and need to calculate Ksp for magnesium fluoride at 25 degree celcius given that a solubility of 0.00172g/100ml.

the solution says, [Mg2+(aq)] = [MgF2(aq)]
why is that?

? · Accepted Answer

If the solution says [Mg²⁺(aq)] = [MgF₂(aq)] that is bacause all the MgF₂ that is in solution is dissociated and one gets one Mg²⁺ ion for every formula unit that is dissolved.  

MgF₂ ⇌ Mg²⁺(aq) + 2F⁻(aq)

That doesn't tell you the Ksp (yet).  
From the stoichiometry [F⁻(aq)] = 2 [Mg²⁺(aq)], and we also know (0.00172g/100ml)(1000 ml/L)/(62.3 g/mol) = 2.76 × 10⁻⁴ mol/L

Ksp = [Mg²⁺][F⁻]² = 8.42 × 10⁻¹¹

degraffenreid · Answer

ok, for 9eb74895db75f88f647ca35148c2ce3 you have a optimal solubility of seven.7e9eb74895db75f88f647ca35148c2ce3 g/l. 9eb74895db75f88f647ca35148c2ce3 Bi3+]*[I-> Bi3+ + 9eb74895db75f88f647ca35148c2ce9eb74895db75f88f647ca35148c2ce Ksp = [Bi3+]*[9eb74895db75f88f647ca35148c2ce9eb74895db75f88f647ca35148c2ce]^3 from 7.7e9eb74895db75f88f647ca35148c2ce3 you may calculate the style of molarity (M) of 9eb74895db75f88f647ca35148c2ce3 you may dissolve and from stoichiometry you may make certain the M of your 2 products and pump them for the time of the Ksp equation for 9eb74895db75f88f647ca35148c2ce3. Wash rinse repeat with iron hydroxide.

sangasy · Answer

ok, for 9eb74895db75f88f647ca35148c2ce3 you have a optimum solubility of seven.7e9eb74895db75f88f647ca35148c2ce3 g/l. 9eb74895db75f88f647ca35148c2ce3 Bi3+]*[I-> Bi3+ + 9eb74895db75f88f647ca35148c2ce9eb74895db75f88f647ca35148c2ce Ksp = [Bi3+]*[9eb74895db75f88f647ca35148c2ce9eb74895db75f88f647ca35148c2ce]^3 from 7.7e9eb74895db75f88f647ca35148c2ce3 you are able to calculate the style of molarity (M) of 9eb74895db75f88f647ca35148c2ce3 you are able to dissolve and from stoichiometry you are able to verify the M of your 2 products and pump them throughout the time of the Ksp equation for 9eb74895db75f88f647ca35148c2ce3. Wash rinse repeat with iron hydroxide.

? · Answer

First scale up to grams per liter of solution.  ( .00172 g/l00ml times l0)

      which would be  .0172 g/

Now get into moles of MgF2 per liter.     Moles= grams over grams/mole

                 Moles MgF2l =  .

0172 g over  62 g/mole = 2.77 X l0^-4


The equilibrium is      MgF2(s)  at equilib with    Mg++(aq)  plus   2 F-(aq)

so Ksp expression is          Ksp   =                   (Mg++)(F-)2

notice that for each mole of Mg++ you get two moles of F-

so fill in the molar amounts.    Ksp =   (2.77 x l0^-4) (5.55 x l0^-4)^2  which -  8.53 x l0^-11

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Calculating the solubility product constant, given the solubility?

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