Cliffs AP Chemistry, 3rd Edition

Experiment 16: Analytical Gravimetric Determination

Background:Analytical gravimetric analysis is a method for determining the amount of a given
substance in a solution by precipitation, filtration, drying, and weighing. The steps generally fol-
low the pattern of weighing the sample, dissolving the sample in an appropriate solvent, forming
a precipitate, filtering the precipitate, drying the precipitate and then finally weighing the precip-
itate. From the data obtained and through mass-mass stoichiometry, the nature of the sample can

be determined. In this experiment, a sample of alum, KAl(SO 4 ) 2 ⋅12 H 2 O, will be analyzed for

the sulfate content and compared to the theoretical percent found from the formula.

The precipitate that will be formed in this experiment is barium sulfate. The precipitate is nei-
ther typically “curdy” nor gelatinous, but rather forms very fine crystals.

Scenario:A student obtained a Büchner funnel, a piece of Whatman No. 42 filter paper, and a
filter flask. She then weighed out 1.059 grams of alum and dissolved it in 50.0 mL of distilled
water. She then calculated how much 0.200 M Ba(NO 3 ) 2 she would need to totally precipitate
all of the sulfate ion present in solution. She carefully added twice this amount to the alum so-
lution, stirring constantly. She then heated the solution to just under the boiling point for 15
minutes and then allowed the solution to stand overnight. The next day she discovered that fine
crystals had appeared in the solution. She weighed the filter paper and determined its mass on
an analytical balance as 1.675 g. She then filtered the solution through the Büchner funnel con-
taining the filter paper and collected the crystals. The filter paper was removed and allowed to
dry in a drying oven set at 50°C to avoid charring of the paper. She returned later and weighed
the paper and crystals and determined their total mass to be 2.715 grams.

Analysis:

1. Calculate how much 0.200 M Ba(NO 3 ) 2 would be needed to totally precipitate all of the
   sulfate ion present in the alum solution.
   .
   .

g KAl SO H O

g KAl SO H O

mol KAl SO H O

1

1 059

474 46

12 1

12

4 2 2 12

4 2 2

4 2 2

#

:

:

^ :

^

h ^

h

h

g KAl SO H O.

mol Ba NO

mol Ba NO

LBa NO

L

mL

1

2

0 200

1

1

1000

12

2

2

2

4 2 2

3

3

3

###

^ :

^

^

^

h

h

h

h

= 22.3 mL

Part III: AP Chemistry Laboratory Experiments