Page 71 - [B._MURPHY,_C._MURPHY,_B._HATHAWAY]_A_working_meth

Equilibrium I. 55

ADVANCED EQUILIBRIUM PROBLEMS-
WORKING METHOD

In the previous section, buffer type problems were introduced, where
the equilibrium expression involved K,, the dissociation constant of an
acid:
i.e. HA + H20 + H30+ + A-, Ka = {[H30'][A-]}/[HA],
from which the pH of the buffer solution could be determined, using
the equation pH = -loglo[H30+]. The following working method
deals with acid/base equilibrium type problems, where it has to be
determined whether (a) acid or base dissociation (K, or Kb) occurs, or
(b) anion or cation hydrolysis (Kh) occurs. Such problems are more
difficult and need to be divided into a series of steps:

1. Read the question carefully.
2. Identify the type of reaction, i.e. acid/base.
3. Identify the type of reactants:

3. HC104 4. HZS04
2. Weak acidi: 1. HN02 2. HC102 3. Carboxylic acids: RC02H
3. Strong bases: 1. NaOH 2. KOH
4. Weak bases: 1. NH3 2. Arnines: 1" (RNHZ), 2" (R2NH), 3" (R3N)
4. Write a balanced equation for the reaction. A Bronsted-Lowry
acid (HfA-) is a proton donor (Hf) and a Bronsted-Lowry
base (B) is a proton acceptor:
i.e. HA + B -+ BH+ + A-
Acid Base Cation Anion
(donates H+) (accepts H +)
5. Determine the amount of each reactant (expressed in moles)
present initially (the amount of product is zero at this stage).
bount (in moles) = [Volume (in cm3)x Molarity (in M)]/1000( (*)

6. Determine the amount (in moles) of reactants and products at
the end of the reaction.
7. Calculate the concentration (i.e. molarity) of all species left in
solution, from (*), i.e. Molarity (in M) = [Amount (in moles) x
1000]/Volume (in cm3)]. However, the volume now involved is the
total volume!
8. Determine next which of these species has the greatest concentra-
tion. This then undergoes the equilibrium reaction: