Chemistry Practice Problems Selective Precipitation Practice Problems Solution: Will a precipitate form when 10 mL of 0.015 M [Hg ...

Solution: Will a precipitate form when 10 mL of 0.015 M [Hg +] and 10 mL of 0.015 M [SO 42−] are mixed? Ksp of Hg2SO4 is 6.5x10−7.a. Yes; Q = 3.4 × 10 −6 which is larger than Ksp, a precipitate will form.b. No; Q = 3.4 × 10 −6 which is larger than Ksp, a precipitate will not form.c. Yes; Q = 4.2 × 10 −7 which is smaller than Ksp, a precipitate will form.d. No; Q = 4.2 × 10 −7 which is smaller than Ksp, a precipitate will not form.e. No; Q = 3.4 × 10 −12 which is smaller than Ksp, a precipitate will not form.

Problem

Will a precipitate form when 10 mL of 0.015 M [Hg +] and 10 mL of 0.015 M [SO 42−] are mixed? Ksp of Hg2SO4 is 6.5x10−7.

a. Yes; Q = 3.4 × 10 −6 which is larger than Ksp, a precipitate will form.

b. No; Q = 3.4 × 10 −6 which is larger than Ksp, a precipitate will not form.

c. Yes; Q = 4.2 × 10 −7 which is smaller than Ksp, a precipitate will form.

d. No; Q = 4.2 × 10 −7 which is smaller than Ksp, a precipitate will not form.

e. No; Q = 3.4 × 10 −12 which is smaller than Ksp, a precipitate will not form.

Solution

We’re being asked to determine if a precipitate will form when 10 mL of 0.015 M Hg+ and 10 mL of 0.015 M SO42– are mixed.


For this, we need to compare the reaction quotient (Q) vs. the solubility product constant (Ksp)

Recall that when:

• Q > Ksp: the solution is supersaturated and a precipitate will form. Reactants are favored.

• Q = Ksp: the solution is at equilibrium and no precipitate will form.

• Q < Ksp: the solution is unsaturated and no precipitate will form. Products are favored.


The expected precipitate is Hg2SO4. The sulfate ion, SO42–, has a charge of –2. Hg then has a charge of +1. 

The dissociation of Ag2SO4 in water is as follows:

Hg2SO4(s)  2 Hg+(aq) + SO42–(aq)


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