Page 64 - [B._MURPHY,_C._MURPHY,_B._HATHAWAY]_A_working_meth

48 Chapter 5
where Kb = ([CH3NH3 +][OH-])/[CH3NH2] is the equilibrium con-
stant for base dissociation.
Table 5.1 summarises some common strong and weak acids and
bases, which need to be identified in problems (R = an alkyl group
e.g. CH3, CH3CH2 etc.)

Table 5.1 Some examples of strong and weak acids and bases
Strong acids: 1. HCl 2. HN03 3. HC104 4. H2S04
Weak acids: 1. HN02 2. HC102 3. Carboxylic acids: RC02H
Strong bases: 1. NaOH 2. KOH
Weak bases: 1. NH3 2. Amines: 1" (RNH2), 2" (RzNH), 3" (R3N)

COMMON ION EFFECT
A common ion is an ion (charged species) common to two substances
in the same mixture, e.g. in a solution of ethanoic acid, CH3C02H and
sodium ethanoate, CH3C02Na, the common ion is the ethanoate
anion, CH3CO:. The common ion effect occurs when the presence of
extra (common) ions in the solution represses or restrains the dissocia-
tion of a species. To explain this, consider the following example:

Example: A solution is prepared by adding 0.6 moles of sodium
ethanoate, CH3C02Na, and 0.8 moles of ethanoic acid, CH3C02H,
to water, to make up 1 dm3. Determine the concentration of all
solute species, given that Ka(CH3C02H) = 1.8 x at 25 "C.

Solution: To solve this problem, the working method of Chapter 4 is
applied.
1. Read the question Carefully-K, problem!
2. Species present in solution: CH3C02Na(,,), CH3CO,H(,,) and
the corresponding ions (step 3 below).
3. (a) CH3C02Na(,,) is a salt and undergoes 100% dissociation into
its anion and cation, CH3C02Na(,,) + Na+(,,) + CH,CO,(aq),
i.e. the equilibrium lies completely to the ionic products!
(b) CH3C02H(,,) is a weak acid. Weak acids do not dissociate
completely into anions and cations. Hence, the reaction is at
equilibrium, i.e. CH3C02H + H20 + CH3C0; + H30+.

In this solution, both reactions (a) and (b) occur simulta-
neously, and so both must be considered when calculating the