http://www.ck12.org Chapter 19. Equilibrium
The Ks pexpressions in terms of s can be used to solve problems in which the Ks pis used to calculate the molar
solubility as in the examples above. Molar solubility can then be converted to solubility in g/L.
Predicting Precipitates
Knowledge of Ks pvalues will allow you to be able to predict whether or not a precipitate will form when two
solutions are mixed together. For example, suppose that a known solution of barium chloride is mixed with a known
solution of sodium sulfate. Barium sulfate (Figure19.5) is a mostly insoluble compound that could potentially
precipitate from the mixture. However, it is first necessary to calculate the ion product, [Ba^2 +][SO 42 −], for the
solution. If the value of the ion product is less than the value of the Ks p, then the solution will remain unsaturated.
No precipitate will form because the concentrations are not high enough to begin the precipitation process. If the
value of the ion product is greater than the value of Ks p, then a precipitate will form. The formation of the precipitate
lowers the concentration of each of the ions until the ion product is exactly equal to Ks p, at which point equilibrium
is attained.
FIGURE 19.5
Barium sulfate is used as a component of
white pigments in paints and as an agent
in certain x-ray imaging processes.Sample Problem 19.3: Predicting Precipitates
Will a precipitate of barium sulfate form when 10.0 mL of 0.0050 M BaCl 2 is mixed with 20.0 mL of 0.0020 M
Na 2 SO 4?
Step 1: List the known quantities and plan the problem.
Known
- concentration of BaCl 2 solution = 0.0050 M
- volume of BaCl 2 solution = 10.0 mL
- concentration of Na 2 SO 4 solution = 0.0020 M
- volume of Na 2 SO 4 solution = 20.0 mL
- Ks pof BaSO 4 = 1.1× 10 −^10 (Table19.5)
Unknown
- value of [Ba^2 +][SO 42 −]
- if a precipitate forms
The concentration and volume of each solution that is mixed together must be used to calculate the values of [Ba^2 +]
and [SO 42 −]. Each individual solution is diluted when they are mixed together. The ion product is calculated and
compared to the Ks pto determine whether a precipitate forms.
Step 2: Solve.