Page 66 - A Working Method Approach For Introductory Physical Chemistry Calculations

50 Chapter 5

= 0.00379 M = [CH3C02H], and the % acid dissociation =
(0.00379/0.8)% = 0.47%.
Therefore, the presence of the extra source of CH3C0, ions
repressed the dissociation of ethanoic acid from 0.47% to
0.003%. This can be explained by Le Chiitelier's Principle, as
discussed in Chapter 4. The addition of ethanoate ions causes the
equilibrium to shift to the left, increasing the concentration of
ethanoic acid, CH3C02H, and reducing the concentration of
H30
+ , i.e. increasing the pH !
i.e. CH3C02H + H20 * CH3CO; + H30+

DISSOCIATION OF HzO AND pH

Dissociation of HzO

Water undergoes self-dissociation generating H30+ and OH- in low
concentration, according to the equilibrium reaction: H20 + H20 +
H30+ + OH-, where K, = ([H30'][OH-]>/([H20][H20]).
But,
since the activity, a, of pure water is unity, the equilibrium can be
described by Kw = [H30+][OH-], where Kw has a value of and
is termed the dissociation constant of water.

PH
The pH (the power of the hydronium or hydrogen ion concentration)
is defined as the log to the base 10 of the hydronium ion concentration
and is a means of expressing the acidity or basicity of a solution:

pH = -loglo[H30+] or pH = -loglo[H+]
Similarly, an expression for the OH- ion can be defined as
pOH = -loglo[OH-]
and remembered by definition as: pH + pOH = 14.

For example, the pH of a 0.15 M solution of HN03 = -loglo[H30+]
=-logl0[0.15] = 0.82. Likewise, the pH of a 0.001 M solution of
NaOH = 14-pOH = 14-(-loglo[OH-1) = 14-3 = 11.
The pH scale is a scale ranging from 0 to 14, with pure deionised water
having an intermediate value of 7.0. The scale is shown in Figure 5.1. It is
not necessary to remember the exact values, just the relative positions
on the scale of both strong and weak acids and bases respectively.