17.4.1 The Solubility Product Constant

Question 1

The Solubility Product Constant

Answer 1

• The solubility product constant, K sp , governs the equilibria of sparingly soluble salts. Given
A x B y (s) <==> xA+(aq) + yB–(aq), K sp = [A+] x [B–] y
• Since they have a fixed value, solids do not appear in solubility product equations.
• The concentrations of individual ions are expressed as the power of their stoichiometric coefficients in the K sp equation.
• If the concentrations of the ions in a saturated solution are known, the K sp can be calculated

Question 2

note

Answer 2

• Let S = [Ag + ] = [Cl – ]
• When S moles/liter of silver chloride are dissolved in water there will be S moles/liter of ions of each Ag + and Cl – in the solution.
• So, given a saturated solution, 1.3 x 10 –5 moles AgCl would dissolve in a liter of water at 25°C.
• For every mol/L (S) of Ca 3 (PO 4 ) 2 there will be
  3 mol/L Ca2+ (3S) and 2 mol/L PO43– (2S).
• Calcium phosphate results from one of the reactions in a water softener. Hard water contains calcium or magnesium ions.
• Water softeners often contain sodium phosphate (Na 3 PO 4 ). The phosphate reacts with the calcium ions and precipitates out the very insoluble calcium phosphate.
• If the ionic concentrations in a saturated solution are known, it is easy to compute K sp .
• Given A x B y (s) <==> xA + (aq) + yB – (aq)
• K sp = [A + ] x [B – ] y

Question 3

Which statement about the Ksp for an equilibrium reaction is not correct?

Answer 3

Ksp values pertain to salts which do not dissociate in aqueous solution.

Question 4

Silver sulfide, Ag2S, has a solubility product, Ksp, of 8 × 10^−51 at 25°C. What are the equilibrium concentrations of Ag2S, Ag+, and S2− in an aqueous solution?

Answer 4

[Ag2S] = 1.3 × 10−17 M; [S2− ] = 1.3 × 10−17 M; [Ag+ ] = 2.6 × 10−17 M

Question 5

What is the solubility of CuS in aqueous solution if Ksp for the dissolution of CuS is 3.2 × 10−28?

Answer 5

1.8 × 10^−14 M

Question 6

How many moles of AgBr would dissolve in 1 L of water if Ksp for AgBr is 5.0 × 10^−13?

Answer 6

7.1 × 10^−7 M

Question 7

What is the solubility of iron(II) hydroxide, Fe(OH)2, if Ksp for
Fe(OH)2 is 7.9 × 10^−16?

Answer 7

5.8 × 10^−6 M

Question 8

What distinguishes the solubility product, Ksp, from other equilibrium constants?

Answer 8

It deals specifically with sparingly soluble salts.

Question 9

Look at the following hypothetical dissociation reaction.

A2B4(s) <==> 2A+(aq) + 4B-(aq)

Which of the following expresses the correct reaction for the Ksp?

Answer 9

Ksp = [A+ ]2 [B− ]4

Question 10

What is the solubility, S, of HgS in aqueous solution if Ksp for the dissolution of HgS is 7.9 × 10^−37?

Answer 10

8.9 × 10^−19 M

Question 11

Silver iodide, AgI, is a sparingly soluble salt that has a Ksp of 8.3 × 10^−17. How many moles of AgI would dissolve in 1 L of water?

Answer 11

9.1 × 10^−9 M

Question 12

Which of the following statements does not accurately describe the solubility product, Ksp?

Answer 12

It’s the equilibrium constant for the dissolution or dissociation of water, a base, or an acid into its component ions.