CHM 2046C Sample Module 12 Name: _Answers____

Part A. The Common-Ion and the Common-Ion Effect 10 points

1. What is a common-ion?

A common ion is an ion, often the conjugate base of a weak acid, the conjugate acid of a weak base or an ion of a sparingly soluble salt, that is the same as an ion already present in the solution as a participant in an equilibrium.

2. What would be the common ion for the following weak acids and weak bases and in addition to its formula give its name:

(a) Lactic Acid: CH₃CH(OH)COOH

Lactate Ion: CH₃CH(OH)COO^1-

(b) Acetic Acid: CH₃COOH

Acetate Ion: CH₃COO^1-

(c) Ammonia dissolved in Water: NH₃ in H₂O

Ammonium Ion: NH₄¹⁺

3. What is the Common-Ion Effect for Weak acids, Weak base, or Salt Solubility?

The common-ion effect refers to the effect the common ion has upon the equilibrium. According the LeChatelier’s Principle, if the concentration of the common ion is increased (more is added) then the system will adjust itself to accommodate the change and maintain the value of the equilibrium constant.

Discuss Examples in General:

The ionization of the weak acid (or base), or the solubility of a salt, will be suppressed:

a) Adding the conjugate base (or conjugate acid) to a solution of an acid (or base) increases the concentration of the acid (or base).

b) Adding a common ion to a saturated solution of a salt will lower the salt solubility.

4. Does the pH of the solution increase, decrease, or stay the same when you:

(a) Add Ammonium nitrate to a dilute aqueous solution of ammonia?

Decrease pH

(b) Add solid potassium acetate to a dilute aqueous solution of acetic acid?

Increase pH

(c) Add solid sodium lactate to a dilute aqueous solution of lactic acid?

Increase pH

(d) Add solid Potassium nitrate to a dilute solution of Potassium hydroxide?

No Effect