To: Ms. Lummis
From: James Gallicchio
Date: 13 October 2016
Re: Gravimetric Analysis of a Metal Carbonate Lab Report
This experiment was designed to demonstrate how gravimetric analysis of a compound
can be used to determine the chemical makeup of the compound. Gravimetric analysis is a
process involving weighing a compound to determine its chemical makeup, using molar massthe mass of 6.02 * 1023 molecules of a compound. Molar mass is simply a measure of the
weight of one molecule but on a scale that is easier to work with. At the start of the lab, we
knew only that the unknown was a metal carbonate, with a formula of M2CO3 (the M standing
for a metal). We dissolved this compound in water (Eq. 1), and then dissolved calcium chloride
in the solution as well (Eq. 2). This prompted a reaction between the ions to form calcium
carbonate as solid precipitate which settled to the bottom (Eq. 3). Calcium carbonate, unlike
the metal carbonate, would form a precipitate in solution rather than remaining suspended
because calcium carbonate is not soluble in water.
Equation 1: M2CO3 → 2M+ (aq) + CO32- (aq)
Equation 2: CaCl2 → Ca2+ (aq) + 2Cl- (aq)
Equation 3: Ca2+ + CO32- → CaCO3 (s)
Before dissolving the unknown in water, and after the precipitate formed, the solid
needed to be dried completely so that the material could be weighed accurately- water is a
heavy compound, and even a small amount can interfere with measurements. To do this, we
used a sand bath, in our lab a pan of laboratory grade sand sitting on a hot plate, and a small
ceramic crucible to hold the compound. For the precipitate, we first poured off excess liquid,
and then filtered the precipitate to remove most of the liquid before drying it in a drying oven
over a few days (see procedure below). Once our data was collected (see table below) we
were able to make our calculations to determine the identity of the metal carbonate (see
calculations below).
Procedure
We began by measuring the crucible which would eventually hold the compound so that
the weight of the compound could afterwards be determined by subtracting the weight of the
crucible from the weight of the crucible and compound. This crucible had previously been
dried thoroughly so the measurement was accurate enough for this lab. Then, we proceeded
to measure out about two grams of the unknown compound. To fully dry the compound (and
thus obtain an accurate measurement) we heated the crucible and compound in the sand bath
for about five minute intervals. Between intervals, we would measure the weight of the
compound until the change between measurements was minimal- this indicated that the
compound was nearly fully dried and dry enough for our purposes.
With an accurate measurement of the amount of the unknown, we then dissolved the
unknown in 200 mL of distilled water in a 400 mL glass beaker, stirring until it was fully
dissolved. 125 mL of calcium chloride was added to the beaker to cause the reaction to occur
and form the precipitate. The mixture was stirred to assure that most of the reactants in the
beaker reacted, and then the precipitate was allowed to settle at the bottom overnight. The
next day, we folded a filter paper into a cone shape and weighed the filter paper. Then, we
placed it in a funnel. The bottom of the funnel drained into another beaker to catch the water
and metal chloride solution. The filter paper was dampened with distilled water to hold it
open against the funnel’s surface. Once this setup was complete, we decanted the excess
liquid from the top by pouring it into the funnel to catch any precipitate that may still be
suspended, being careful to not cause the precipitate at the bottom to become suspended.
With a small amount of liquid left in the beaker, we swirled the beaker to suspend the
precipitate in liquid. We then proceeded to carefully filter this mixture through the funnel.
After all of this liquid was filtered, a wash bottle was used to wash the sides of the beaker with
a small amount of distilled water, and then this liquid in the beaker was filtered to catch any
remaining precipitate. The filter was then removed from the funnel and carefully placed on a
curved piece of glass called a watch glass. This watch glass with filter was placed in a drying
oven and allowed to dry over the weekend. Finally, we weighed the dried filter paper with
calcium carbonate precipitate. At this point enough data was collected to proceed.
Gravimetric Analysis Data
Mass of crucible + M2CO3
9.629 g
Mass of crucible + M2CO3 (dried) (1st weighing)
9.575 g
Mass of crucible + M2CO3 (dried) (2nd weighing)
9.559 g
Mass of crucible
7.666 g
Mass of M2CO3
1.893 g
Mass of filter paper + CaCO3 (dried)
1.873 g
Mass of filter paper
0.576 g
Mass of CaCO3
1.297 g
Molar mass of CaCO3 (actual)
100.087 g/mol
Moles of CaCO3
0.01296 mol
Molar mass of M2CO3
146.065 g/mol
Molar mass of Li2CO3 (actual)
73.891 g/mol
Molar mass of Na2CO3 (actual)
105.9888 g/mol
Molar mass of K2CO3 (actual)
138.205 g/mol
Identity of M2CO3
K2CO3
Percent error
5.7%
Calculations
The mass of the M2CO3 sample as well as the mass of the CaCO3 sample was calculated
by subtracting the mass of the container (the crucible and the filter paper, respectively) from
the mass of the container and sample.
The molar mass of each known compound was calculated using the atomic mass of each
element added together. For instance, CaCO3’s molar mass is the sum of the atomic mass of
each of its elements- 1 calcium (40.078 amu) + 1 carbon (12.0107 amu) + 3 oxygens (3 *
15.9994 amu) = 100.087 amu. One atomic mass unit (amu) is equal to one gram per mole
(g/mol).
Next, based on the mass of our CaCO3 sample and its molar mass, we were able to
calculate the number of moles of CaCO3 which were produced by the reaction:
1.297 𝑔
100.087
𝑔 = 0.01296 𝑚𝑜𝑙
𝑚𝑜𝑙
Using this measurement, the molar mass of the unknown was calculated (see conclusion):
1.893 𝑔
𝑔
= 146.065
0.01296 𝑚𝑜𝑙
𝑚𝑜𝑙
This was then compared to the calculated molar mass of known metal carbonate
compounds such as Li2CO3, Na2CO3, and K2CO3, and since our experimental molar mass was
closest to the molar mass of K2CO3 we determined the identity of the compound to be
potassium carbonate.
Conclusion
Based on our findings, the unknown metal carbonate was identified as potassium
carbonate (K2CO3). This was the compound with the closest molar mass to that which we
found. The actual molar mass of potassium carbonate is 138.205 g/mol, while the
experimental one was 146.065 g/mol. This means that our experiment had a 5.7% error, but is
close enough to confidently determine the identity of the compound. Moles and molar mass
played a major role in this experiment’s findings. The reaction that produced the precipitate
had a 1:1 ratio of carbonate between the original substance and the precipitate (both had
exactly 1 ion of CO3 per molecule). Thus, the number of moles of calcium carbonate would
equal the moles of unknown substance, assuming enough calcium was available to the
solution so that the carbonate rather than calcium was the limiting factor. By calculating the
moles of calcium carbonate, we then could calculate the moles and eventually the molar mass
of the unknown. Once we knew the molar mass of the unknown, determining its identity was
as simple as comparing it to other known molar masses.