Solutions for Gravimetric Analysis Exercises
1. The terms in a reaction quotient are actually dimensionless ratios of actual
concentrations (or pressures) divided by standard concentrations (or pressures).
The standard state for solutes is a 1 M solution and for gases it is a pressure of 1
bar (~ 1 atm), so these are the units used. For liquids and solids the standard
state is the pure substance, so these ratios are unity. The solvent in a dilute
solution is approximated as a pure substance.
2. The equilibrium constant for a reaction is dimensionless because it equals the
reaction quotient at equilibrium, and, as discussed in Problem 1, this ratio
involves only dimensionless terms.
3. a) Adding the two equations together gives
(1) AgCl(s) º Ag+(aq) + Cl-(aq) K1 = 1.8 x 10-10
(2) Ag+(aq) + Cl-(aq) º AgCl(aq) K2 = 2.0 x 103
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(3) AgCl(s) º AgCl(aq) K3 = K1@K2 = 3.6 x 10-7
b) At equilibrium K3 = [AgCl(aq)], so [AgCl(aq)] = 3.6 x 10-7 M.
4. Determine the solubility of La(IO3)3 using Ksp for La(IO3)3 and a table of initial and
equilibrium concentration terms.
La(IO3)3(s) X La3+(aq) + 3lO3-(aq)
init 0 0.050
equil x 0.050 + 3x
where x equals the increase in the [La3+] as a result of the dissolving process.
at equil, Ksp = 1.0 x 10-11 = [La3+][lO3-]3 = (x)(0.050 + 3x)3 • (x)(0.050)3
assumes 3x << 0.050
(assumption valid)
