Inorganic and Applied Chemistry

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Inorganic and Applied Chemistry

As pH is a logarithmic scale based on the decimal system means that the pH value of a solution is changed

one unit for each time the [H+] is changed 10 times. E.g. does a solution with a pH-value of 4 have a

concentration of H+ ions 10 times as high as a solution with a pH value of 5.

5.1.3 The autoprotolysis of water

As indicated earlier water may function both as an acid and as a base. Such a property is often referred to as

an amfolyte and water is an amfolyte. This means that water may react with it self in the following reaction:

H 2 O(l) + H 2 O(l)  H 3 O+(aq) + OH-(aq)

The equilibrium lies nevertheless far to the left. The equilibrium constant for the specific equilibrium is

referred to as Kw having the following values.

K 1 , 01014 M^2

w

This means that there are only relatively few water molecules that are transferred to H 3 O+ and OH- ions. This

equilibrium sets itself in pure water and in aqueous solutions. In pure water [H 3 O+] = [OH-] as the ions are

produced in an 1:1 relation. At a temperature of 25 ºC in pure water:

[H 3 O+] = [OH-] = 1,0 · 10-7 M

which is why pH thereby in pure water is pH = 7,0. This means that in pure water the concentration of H 3 O+

and OH- respectively is in the order of 10-7 moles/L. If we later in order to determine the concentrations of

H+ or OH- in acid and base solutions calculate concentrations in the range of 10-7 moles/L one may not

neglect the effect of autoprotolysis of water. If instead we calculate concentrations of H+ or OH- in the range

of 10-5 moles/L or larger we will often neglect the effect of autoprotolysis of water.

5.2 pH calculations

We will In the following section look at methods for calculating pH values in a number of different solutions.

5.2.1 Calculation of pH in strong acid solutions

Calculations of pH values in solutions are closely connected with the equilibrium considerations described in

the previous chapter. As it is the case when equilibrium is considered one has to focus on components of the

solution and its chemistry. This will be further looked into in the following example:

Acids and bases