How do you use the K_"sp"Ksp value to calculate the molar solubility of the following compound in pure water?

"BaSO"_4BaSO4

K_"sp" = 1.07xx10^(-10)Ksp=1.07×1010

1 Answer
Jun 8, 2016

Here's how you can do that.

Explanation:

What you need to do here is set up an ICE table based on the equilibrium reaction that describes the way barium sulfate, "BaSO"_4BaSO4, dissolves in aqueous solution.

The salt is considered insoluble in water, so right from the start you know that the solution will contain very little amounts of dissolved ions, since most of the compound will remain undissociated as a solid.

So, barium sulfate will dissolve in very small quantities to produce

"BaSO"_ (4(aq)) rightleftharpoons "Ba"_ ((aq))^(2+) + "SO"_ (4(aq))^(2-)BaSO4(aq)Ba2+(aq)+SO24(aq)

Notice that for every mole of barium sulfate that dissolves in solution, you get one mole of barium cations, "Ba"^(2+)Ba2+, and one mole of sulfate anions, "SO"_4^(2-)SO24.

If you take ss to be the molar solubility of the salt, i.e. the number of moles of barium sulfate that dissolve to release ions per liter of solution, you can use an ICE table to write

"BaSO"_ (4(aq)) rightleftharpoons " ""Ba"_ ((aq))^(2+) " "+" " "SO"_ (4(aq))^(2-)BaSO4(aq) Ba2+(aq) + SO24(aq)

color(purple)("I")color(white)(aaaacolor(black)(-)aaaaaaaaaaacolor(black)(0)aaaaaaaaaacolor(black)(0)Iaaaaaaaaaaaaaaa0aaaaaaaaaa0
color(purple)("C")color(white)(aaaacolor(black)(-)aaaaaaaaacolor(black)((+s))aaaaaacolor(black)((+s))Caaaaaaaaaaaaa(+s)aaaaaa(+s)
color(purple)("E")color(white)(aaaacolor(black)(-)aaaaaaaaaaacolor(black)(s)aaaaaaaaaacolor(black)(s)Eaaaaaaaaaaaaaaasaaaaaaaaaas

By definition, the solubility product constant, K_(sp)Ksp, for this solubility equilibrium will be equal to

K_(sp) = ["Ba"^(2+)] * ["SO"_4^(2-)]Ksp=[Ba2+][SO24]

Since the expression of K_(sp)Ksp uses equilibrium concentrations, you can say that

K_(sp) = s * s = s^2Ksp=ss=s2

In your case, you have

1.07 * 10^(-10) = s^21.071010=s2

Solve for ss to find

s = sqrt(1.07 * 10^(-10)) = 1.03 * 10^(-5)s=1.071010=1.03105

Since ss represents molar solubility, you can say that you have

s = color(green)(|bar(ul(color(white)(a/a)color(black)(1.03 * 10^(-5)"M")color(white)(a/a)|)))

This means that when you add barium sulfate to water, you can only hope to dissolve 1.03 * 10^(-5) moles for every liter of solution.