15.2 Predicting Precipitation
When ionic compounds are added to water, they dissociate and produce ions. These free-floating ions can combine and make a precipitate if the reaction quotient Qsp of the compound produced in the solution is greater than the compound’s Ksp value.
Let’s take an example of two solutions: silver nitrate AgNO3 and Sodium chloride NaCl. The solution of AgNO3 will have silver and nitrate ions, and the solution of NaCl will have sodium and chloride ions.
AgNO3 
Ag+ (aq) + NO3– (aq)
NaCl Na+ (aq) + Cl– (aq)
When these two solutions are allowed to mix, silver and chloride ions can combine to produce silver chloride, and sodium and nitrate ions can combine to produce sodium nitrate. Based on solubility rules, we know, sodium nitrate is highly soluble in water; therefore, it will stay dissociated, and sodium and nitrate ions will still be floating in the solution. However, silver chloride has a small solubility, so depending on the concentration of silver and chloride ions present in the solution, these two ions can combine and form a precipitate.
A formula reaction for the above reaction can be written like this:
AgNO3 + NaCl AgCl (s) + NaNO3(aq)
A complete ionic equation will look like this
AgNO3 + NaCl AgCl (s) + NO3– (aq) + Na+ (aq)
And, a net ionic equation will be
Ag+ (aq) + Cl– (aq) AgCl (s)
We can find the concentration of silver ions from the concentration of AgNO3 solution, and we can find the concentration of chloride ions from the concentration of NaCl solution.
We can calculate the Qsp by using this expression
Qsp = [Ag+] [Cl–]
Recall your knowledge from the equilibrium chapter, when Qsp = Ksp system is at equilibrium. Any increase of concentration of either silver or chloride ions will result in the formation of AgCl precipitate. When Qsp > Ksp, equilibrium will move in the forward direction, a precipitate will form.
Example 5: Will a precipitate form if 1.0 L of a 0.15 M NaCl solution was allowed to mix with 1.0 L of 0.15 M AgNO3 solution?
Initially, the solution of NaCl will have Na+ and Cl– ions as per the reaction below.
NaCl 
Na+ (aq) + Cl– (aq)
The concentration of both Na+ and Cl– ions in this solution will be 0.15 M.
And the solution of AgNO3 will have Ag+ and NO3– ions as per the reaction below.
AgNO3 Ag+ (aq) + NO3– (aq)
In this solution concentration of both Ag+ and NO3– ions will be 0.15 M.
Since we are mixing 1.0 L of NaCl with 1.0 L solution of AgNO3, the final volume of solution will be 2.0 L. To calculate the final concentration of ions, we can use the dilution formula.
C1V1 = C2V2
Here C1 is the initial concentration and V1 is the initial volume of solution and C2 is the final concentration and V2 is the final volume of solution.
C2 = 
The final concentration of Ag + will be C2 = 
= 0.075 M
Similarly, the final concentration of Cl– will be C2 = 
= 0.075 M
Qsp = [Ag+] [Cl–]
Qsp = 0.075 x 0.075 = 0.0056
From the reference table above, Ksp for AgCl is 1.6 x 10-10, Since Qsp > Ksp a precipitate will form.
Watch this YouTube video below for an illustration of the same concept.
https://www.youtube.com/watch?v=E1WD5BQ6MRI
A real-life example of precipitation reactions can be seen in the formation of kidney stones. Calcium oxalate monohydrate CaC2O4·H2O is one of the main salts present in kidney stones. When its Qsp value is greater than the Ksp value, it starts depositing in kidneys in the form of a precipitate, which we call kidney stones.
Learning Check:
- Will a precipitate of BaSO4 form when the 15.0 ml of 0.0015 M Ba(NO3)2 solution was allowed to mix with 15.0 ml of 0.0015 M H2SO4 solution? Ksp for BaSO4 = 1.5 x 10-9
- Find Qsp when the 15.0 ml of 2.5 x 10-5 M Ba(NO3)2 solution was allowed to mix with 15.0 ml of 3.5 x 10-5 M H2SO4 solution?
- Will a precipitate of BaSO4 form in this solution, given Ksp for BaSO4 = 1.5 x 10-9.
Answer: 6) Yes, a precipitate will form Qsp > Ksp
7) Qsp = 2.19 x 10-10
8) No, a precipitate will not form Qsp < Ksp