Page 54 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

2 RNDAMENTAL THEORETICAL PRINCIPLES OF REACTIONS IN SOLUTION

Example 2. Calculate the solubility product of silver chromate, given that its
solubility is 2.5 x 10 -' g L-'.

The relative molecular mass of Ag2Cr04 is 331.7; hence the solubility=
2.5 x 10-2/331.7 = 7.5 x IO-' mol L-'.
Now 1 mole of Ag2Cr04 gives 2 moles of Ag+ and 1 mole of CrOz-;
therefore

Example 3. The solubility product of magnesium hydroxide is 3.4 x
IO-" mol3 L-3. Calculate its solubility in grams per L.

The relative molecular mass of magnesium hydroxide is 58.3. Each mole of
magnesium hydroxide, when dissolved, yields 1 mole of magnesium ions and 2
moles of hydroxyl ions. If the solubility is xmolL-', [Mg2+] =x and
[OH-] = 2x. Substituting these values in the solubility product expression:

or x = 2.0 x mol L-'

The great importance of the solubility product concept lies in its bearing
upon precipitation from solution, which is, of course, one of the important
operations of quantitative analysis. The solubility product is the ultimate value
which is attained by the ionic concentration product when equilibrium has been
established between the solid phase of a difficultly soluble Salt and the solution.
If the experimental conditions are such that the ionic concentration product is
different from the solubility product, then the system will attempt to adjust itself
in such a manner that the ionic and solubility products are equal in value. Thus
if, for a given electrolyte, the product of the concentrations of the ions in solution
is arbitrarily made to exceed the solubility product, as for example by the
addition of a Salt with a common ion, the adjustment of the system to equilibrium
results in precipitation of the solid salt, provided supersaturation conditions
are excluded. If the ionic concentration product is less than the solubility product
or can arbitrarily be made so, as (for example) by complex Salt formation or by
the formation of weak electrolytes, then a further quantity of solute can pass
into solution until the solubility product is attained, or, if this is not possible,
until al1 the solute has dissolved.

2.7 QUANTITATIVE EFFECTS OF A COMMON ION
An importaot application of the solubility product principle is to the calculation
of the solubility of sparingly soluble salts in solutions of salts with a common