Laboratory 07
GRAVIMETRIC ANALYSIS : CALCIUM CARBONATE
IN TEXAS LIMESTONE
Objectives
- Use an understanding of solubility rules and
precipitation reactions to isolate calcium from a real
world sample
- Use mass relationships to determine the percent
composition of calcium carbonate in limestone
Background
An understanding of the properties of a substance
begins with the determination of its elemental
composition
Percent composition of a pure substance can be
determined from the formula :
% Composition =
(mass of element)
X 100
(molar mass of compound)
Theoretical percent composition
The percentage composition of an impure substance can be
expressed as
% Composition = (mass of component of interest) X 100
(total mass of sample)
Gravimetric Analysis
- Add a compound ( precipitating agent ) to a
dissolved sample that combines selectively with
the element of interest ( the analyte), causing it to
form a solid precipitate of known identity and
composition in order to identify what elements are
present, and in what quantities.
- Isolate and weigh the precipitate to quantify the
analyte
Gravimetric analysis is used to determine the amount
of calcium carbonate ( CaCO ) in a sample of limestone
3
IMPORTANT:
Do TWO trials in PARALLEL for the whole experiment
Changes to procedure:
Determination of carbonate in the Limestone
1.The limestone has already been broken into
smaller pieces (Use about 1.5 g)
2. Dissolve the limestone in HCl. (10 – 15 minutes )
CaCO3 (s) + 2 HCl (aq)  CaCl2 (aq) + H2CO3 (aq)
H2CO3 (aq)
 CO2 (g) + H2O (l)
CaCO3 (s) + 2 HCl (aq)  CaCl2 (aq) + CO2 (g) + H2O (l)
3. Do gravity filtration using Whatman Filter paper 1.
(15-20 minutes) - No gooch crucible
Determination of calcium in the Limestone
1. About 5-6 drops of indicator (kept in stopper bottles) are
enough to get the pink color.
2. After adding 3 grams of (NH4)2SO4 it is hard to see it
dissolving as very quickly the precipitate appears and the
solution now looks baby pink in color.
(Ensure that (NH4)2SO4 is completely dissolved and no
lumps are left behind)
CaCl2 (aq) + (NH4)2SO4 (aq)  CaSO4 (s) + 2 NH4Cl (aq)
3. Add about 20-24 ml NH3 solution. The color changes to
a very very pale yellow. (Do this step in a hood)
4. Use vacuum filtration and Whatman Filter paper 2.
(No gooch crucible)
Weigh the empty filter paper + watch glass. Use the same
filter paper for vacuum filtration.
Wash the solid using water.
Transfer the filter paper along with the precipitate to the
watch glass and keep it for drying until the next lab in the
drawer.
Finish the calculation and turn in the lab report on next
lab.
Reflection Questions
1, 2 , 3 and 4 (should be answered at home)
5 (should be answered in the next lab)
Grading
Deviation
± 10%
± 12.5%
± 15%
± 20%
± 25%
Points deducted
None
2 points
5 points
10 points
15 points