Practice Problem Set Oct. 21 2013
Answer the Following Questions:
1. Calculate the increase in percent dissociation of 0.1M AB2 in a solution with an
ionic strength equals 0.09, as compared to dissociation in water. The equilibrium
is shown below:
AB2 A2+ + 2BThe thermodynamic equilibrium constant is 3.0*10-7, assume that the effective
diameters of hydrated A+ and B- is equal to 3Ao, each.
a. In the solution with ionic strength of 0.09
- log fi = (0.51 Zi2 µ1/2)/(1 + µ1/2)
For A2+ we have
- log fA2+ - log fB- = (0.51 x 22 * 0.091/2)/(1 + 0.091/2) = 0.47
fA2+ = 10-0.47 = 0.34
For B- we have
- log fB- = (0.51 x 12 * 0.091/2)/(1 + 0.091/2) = 0.118
fB- = 10-0.118 = 0.76
KTh = {[A2+][B-]2/[AB2]} {fA2+ (fB-)2 /fAB2}
3.0*10-7 = x*(2x)2/(0.1-x) {0.34*(0.76)2/1}
X = 1.14*10-3
RE = 1.1%
Assumption is valid
%dissoc. = 1.14%
b. In pure water
3.0*10-7 = x*(2x)2/(0.1-x)
X = 1.96*10-3
RE = 1.96%
Assumption is valid
%dissoc. = 1.96%
% increase in dissoc. = {(1.96-1.14)/1.14}*100 = 72%
2. Circle the correct term(s), in italics, in each of the following sentences:
a. Gravimetric methods are classified as macro ,
micro or
ultramicro
b. The supersaturation ratio in a gravimetric procedure should be kept at a
maximum
minimum
c. Precipitation of an analyte should be performed using hot , cold, dilute,
concentrated solutions
d. Gravimetry is one of the most accurate
inaccurate analytical
where it requires accurately prepared standards
e. A large excess
methods
no standard solutions
A small excess of precipitating agent should always
be used in precipitation using gravimetry.