MCAT Content / Solubility / Solubility Constant The Equilibrium Expression Ksp

Solubility Constant; The Equilibrium Expression Ksp

Topic: Solubility

The equilibrium expression, Ksp, is a ratio of products over reactants and can be written as Ksp = [products]/[reactants]. This expression represents the equilibrium between an ionic solid and its ions in solution.

In a reaction where a chemical is dissolved into separate components in a solution (such as water), there is a natural tendency for the reaction to reach equilibrium, which is a state in which the products and reactants are in balance with each other. The tendency for a compound to reach equilibrium in solution is represented by the equilibrium expression, Ksp. The expression can be written out as the concentration of the products of the reaction over the concentration of reactants in the reaction:

Ksp = [products]/[reactants]

This expression uses the molarity (M), or concentration, of the products and reactants. Concentrations, in this case, are noted by the use of brackets. For example, let us say we have a compound, Ag2CrO4(s), and we completely dissolve it in water. This can be represented by the following equilibrium reaction:

Ag2CrO4(s) –> 2 Ag+(aq) + CrO4-(aq)

We are given the concentration of Ag2CrO4(s), which is 7.8*10-5 M. Some important notes to remember when solving for Ksp:

• Pure solids (s) and liquids (aq) are not included in equilibrium expressions. Aqueous (aq) and gas (g) species are included.
• When a species has a coefficient, the concentration of that species is both raised to the power of the coefficient value and multiplied by the coefficient value.
• If there are multiple products or reactants, multiply them by each other in the expression. See the example below.
• If it is explicitly stated that the species fully dissociates in solution, you can assume that is concentration is also the concentration of the individual species.

With these points in mind, we can write an expression for the Kspof Ag2CrO4(s):

Ksp = (2*[Ag+])2 [CrO4]

There is nothing in the denominator (bottom of the fraction) of the expression because the reactant of the reaction, Ag2CrO4(s), is solid. Plugging in the given molarity we are given the following equation to solve:

Ksp = (2*7.8*10-5 M)2*(7.8*10-5 M)

Ksp = 1.90*10-12

The final value for Ksp is unitless. Because the value of Ksp is small, we can assume Ag2CrO4(s) has a low solubility value.

Practice Questions

MCAT Official Prep (AAMC)

Chemistry Online Flashcards Question 2

Chemistry Question Pack Question 92

Chemistry Question Pack Passage 17 Question 96

Chemistry Question Pack Passage 17 Question 98

Sample Test C/P Section Passage 1 Question 3

Practice Exam 2 C/P Section Passage 8 Question 43

Practice Exam 3 C/P Section Passage 10 Question 53

Key Points

• The expression for the solubility constant in equilibrium is Ksp = [products]/[reactants].

• Pure solids (s) and liquids (aq) are not included in equilibrium expressions. Aqueous (aq) and gas (g) species are included.

• When a species has a coefficient, the concentration of that species is both raised to the power of the coefficient value and multiplied by the coefficient value.

• If there are multiple products or reactants, multiply them by each other in the expression.

• Ksp  is unitless.

• If it is explicitly stated that the species fully dissociates in solution, you can assume that is concentration is also the concentration of the individual species.

Key Terms

Equilibrium/equilibria: the state in which products and reactants are in balance in each other

Molarity: concentration of a substance, denoted with brackets and represented by moles/Liters or M

Concentration: a measure of the concentration of a chemical species in terms of the amount of substance per unit volume of solution

Product: end result of a reaction; the ride side of an equilibrium equation

Reactant: combined with other reactants to form the product; the left side of an equilibrium equation

Billing Information