Ksp - Which will precipitate

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[pvnguyen1]

A 0.012-mol sample of Na2SO4 is added to 400 mL of each of the two solutions.  One solution contains 2.5 x 10^-3 BaCl2; the other contains 2.5 x 10^-3 CaCl2.  (Ksp for BaSO4 = 1.5 x 10^-9; Ksp for CaSO4 = 6.1 x 10 ^ -5).  Calculate to determine which one; CaSO4 or BaSO4 will form precipitate.

[kyle1990]

Determine the molar concentration of each cation in solution:

2.5X10^-3M = Ba ²⁺

the concentrations of Ca2+ is also the same as this

Calculate the molar solubility of each substance from the Ksp.

Ex: Ksp BaSO4= 1.5X10^-9

1.5X10^-9= [Ca²⁺][SO₄^2-]
= (x)(x)
= x²

x= molar solubility of Ba2+ and SO42- in mol/ L

do the same calculation for CaSO4


Then determine the molar concentration of SO42- ions that are being added to each solution. The Na+ ions do not matter, since SO42- is the only ion that is precipitating.

0.012mol/ 0.400L =3.0X10^-2 mol/L  SO42- (from Na2SO4)


Now we can see which substance will precipitate using Q.

Q= [Ba²⁺][SO₄^2-]

plug in your values for Ba2+ and SO42- from Na2SO4. If Q > ksp, then precipitation occurs. If Q< ksp, then the solution is either at or below the saturation point. Perfrom the same calculation for the Ca2+ ion as well