By Verallo, Breeza Marie
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
mass- the mass of 6.02 x 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.
2
At the start of the lab, the unknown is a metal carbonate, with a formula of
M2CO3 (the M standing for a metal).
When 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)
3
1.
Measure 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. The crucible has to be dried thoroughly
so the measurement is accurate enough for this lab.
2.
Measure out about two grams of the unknown compound. To fully dry the compound
(and thus obtain an accurate measurement) the crucible is heated with the compound in
the sand bath for about five minute intervals. Between intervals, measure the weight of
the compound until the change between measurements is minimal- this indicates that
the compound is nearly fully dried and dry enough for purposes.
3.
Dissolve the unknown in 200 mL of distilled water in a 400 mL glass beaker, stirring
until it is fully dissolved.
4.
Add 125 mL of calcium chloride to the beaker to cause the reaction to occur and form
the precipitate.
5.
Stir the mixture to assure that most of the reactants in the beaker reacted, and then the
precipitate is allowed to settle at the bottom overnight.
4
6.
The next day, fold a filter paper into a cone shape and weighed the filter paper. Then,
place it in a funnel. Drain the bottom of the funnel into another beaker to catch the water
and metal chloride solution.
7.
Damp the filter paper with distilled water to hold it open against the funnel’s surface.
8.
Once this setup is complete, decant 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.
9.
With a small amount of liquid left in the beaker, swirl the beaker to suspend the
precipitate in liquid. Carefully filter this mixture through the funnel. After all of this liquid
is filtered, use a wash bottle to wash the sides of the beaker with a small amount of
distilled water, and then filter the liquid in the beaker to catch any remaining precipitate.
10. Remove the filter from the funnel and carefully place on a curved piece of glass called a
watch glass. Place the watch glass with filter in a drying oven and allow it to dry over
the weekend.
11. Finally, weigh the dried filter paper with calcium carbonate precipitate.
5
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%
6
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).
7
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.
8
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.
9
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.
10
References:
▪
▪
▪
Chemlibretext. (2020). Gravimetric Analysis. Retrieved from
https://chem.libretexts.org/Bookshelves/Ancillary_Materials/Laboratory_Experimen
ts/Wet_Lab_Experiments/General_Chemistry_Labs/Online_Chemistry_Lab_Manual/C
hem_11_Experiments/07%3A_Gravimetric_Analysis_(Experiment)
Flinn Scientific. (2016). Gravimetric Analysis of a Metal. Retrieved from
https://www.flinnsci.com/api/library/Download/a3a20bb3e1774ad59bac39f191821b75
Gallicchio, J., (2016). Gravimetric Analysis of a Metal Carbonate. Retrieved from
https://webcache.googleusercontent.com/search?q=cache:t1OW8mDnyvgJ:https://
www.wtps.org/cms/lib/NJ01912980/Centricity/Domain/875/Gravimetric%2520Lab%
2520Report%2520Example%2520JG.docx+&cd=17&hl=en&ct=clnk&gl=ph
11
End of Slide
12