Untitled - Kelly Walsh High School

240 CHEMISTRY FOR THE UTTERLY CONFUSED
Quick Tip
b. 1.00 M Na 2CrO 4 l 1.00 M CrO 4 2 (common ion)
Ag 2CrO 4(s) K 2 Ag (aq) CrO 4 2 (aq)
2 x 1.00 x
K sp 1.9 10 12 (2 x) 2 (1.00 x) 1.00 (4 x 2 ) 4 x 2
neglecting x
x 6.9 107 [Ag ] 2 x 1.4 10 6 M
Knowing the value of the solubility product constant can also allow us to predict
whether or not a precipitate will form if we mix two solutions, each containing
an ion component of a slightly soluble salt. We calculate the reaction
quotient (many times called the ion product), which has the same form as the
solubility product constant. We take into consideration the mixing of the volumes
of the two solutions, and then compare this reaction quotient to the K sp.
If it is greater than the K sp then precipitation will occur until the ion concentrations
reduce to the solubility level.
If 10.0 mL of a 0.100 M BaCl 2 solution is added to 40.0 mL of a 0.0250 MNa 2SO 4
solution, will BaSO 4 precipitate? K sp for BaSO 4 1.1 10 10
In order to answer this question, we will need the concentration of the barium
ion and the sulfate ion before precipitation. These may be determined from the
dilution relationship:
For Ba
Mdil MconVcon/Vdil. 2 : Mdil (0.100 M) (10.0 mL)/(10.0 40.0)mL 0.0200 M
2 For SO4 : Mdil (0.0250 M) (40.0 mL)/(50.0 mL) 0.0200 M
Inserting these values into the reaction quotient relationship gives:
Q [Ba 2 ][SO 4 2 ] (0.0200)(0.0200) 0.000400 4.00 10 4
4.00 10 4 > 1.1 10 10
Since Q is greater than K sp, precipitation will occur.
A reaction quotient, Q, calculation will always use initial concentrations. There
are never any x values.