Experiment-8: Analysis by Precipitation
Aim: To find out the percent of calcium chloride (CaCl2) in given unknown mixture which is
made-up of two ionic solids, calcium chloride and sodium chloride (NaCl).
Calcium chloride and sodium chloride are water-soluble solids. Calcium chloride reacts with
Sodium carbonate (Na2CO3) in aqueous medium to produce calcium carbonate (CaCO3), white
precipitate (eq.1). Whereas, sodium chloride and sodium carbonate do react (eq.2).
CaCl2 (aq) + Na2CO3 (aq)  CaCO3 (s) + 2 NaCl (aq)
…(eq.1)
NaCl (aq) + Na2CO3 (aq)  No reaction
…(eq.2)
We take the advantage of the fact that only calcium chloride reacts with sodium carbonate and
sodium chloride does not. You will take a known amount of unknown solid, which contains
calcium chloride and sodium chloride, and dissolve in water. Excess of sodium carbonate
solution will be then added to achieve complete precipitation of calcium carbonate, a white
insoluble solid. Using gravity filtration, entire calcium carbonate will be collected on a preweighed filter paper. After oven drying and cooling, you will find the exact mass of calcium
carbonate formed.
The moles of calcium carbonate formed via precipitation reaction in this process are equal to the
moles of calcium chloride present in the unknown mixture (from the stoichiometry of the
balanced chemical equation.1). Moles of calcium chloride then will be converted into grams of
calcium chloride to finally calculate its percentage in the unknown mixture.
Your instructor will demonstrate the proper process of Gravity Filtration.
Observe carefully and follow the instructions!
Procedure
1. Obtain a clean and DRY 250-mL beaker. Find the exact mass of this beaker. Record all
the digits. Label this as #1.
2. Using a dry spatula, transfer approximately 1.5 gram of ‘unknown mixture’ into
beaker#1. Using the same scale, find the exact mass of ‘beaker and unknown mixture’.
Record all the digits. Mass of the ‘unknown mixture’ should not be more than 1.500 g.
3. Slowly add 50 mL of deionsied water into beaker#1. Stir the solution using a glass rod.
Warm up this solution to approximately 50 oC on a hot plate. Stir the solution
occasionally using a glass rod while warming. BO NOT BOIL! DO NOT OVER
HEAT!!. Remove beaker#1 from the hot plate and put it aside.
4. Obtain a clean and DRY 100-ml or 150-mL beaker. Find the exact mass of this beaker.
Record all the digits. Label this as #2.
5. Using a dry spatula, transfer approximately 2 gram of ‘sodium carbonate’ into beaker#2.
Using the same scale, find the exact mass of ‘beaker and sodium carbonate’. Record all
the digits. Mass of the ‘sodium carbonate’ should be 1.8 – 2.2 g range.
6. Slowly add 50 mL deionzied water into beaker#2. Stir the solution using a glass rod.
Warm up this solution to approximately 50 oC on a hot plate. Stir the solution
occasionally using a glass rod while warming. BO NOT BOIL! DO NOT OVER
HEAT!!. Remove beaker#2 from the hot plate and put it aside. Shut-off and unplug the
hot plate.
7. Carefully transfer the liquid solution from beaker#2 into beaker#1. Stir the mixture using
a glass rod. Stir the mixture for about 1 minute using a glass rod. Allow the precipitate to
form for at least 10 minutes.
8. While the precipitation takes place, weigh a piece of filter paper. Record all the digits.
Write your name on it using a pencil. Prepare a glass funnel for gravity filtration (fig.1).
9. Filter the contents of beaker#1 to collect calcium carbonate, the precipitate (fig.2).
10. Place the glass funnel with the filter paper and calcium carbonate inside a preheated oven
(~110 oC) until filter paper becomes crisp dry.
11. Weigh the dry filter paper and calcium carbonate in it.
Disposal: Regular wastes sink/trash bin.
Laboratory Report#8: Analysis by Precipitation
Last Name____________________, first name________________
Date of Experiment___________Partner’s name________________Instructor’s Initials_______
Unknown sample #_______
Data
1. Mass of ‘beaker#1’
=_______________g
2. Mass of ‘beaker#1 and unknown mixture’
=_______________g
3. Mass of ‘unknown mixture’ used [#2-#1]
=_______________g
4. Mass of ‘beaker#2’
=_______________g
5. Mass of ‘beaker#2 with sodium carbonate’
=_______________g
6. Mass of sodium carbonate used(5-4)
=_______________g
7. Mass of ‘filter paper’
=_______________g
After precipitation, filtration, drying and cooling,
8. Mass of ‘filter paper and calcium carbonate
=_______________g
Calculations
9. Mass of ‘calcium carbonate’ formed [#8-#7]
=______________g
10. Moles of calcium carbonate formed (covert #9 into moles) [#9/100.1 g/mol]
=________
11. Moles of calcium chloride present in beaker#1 (= same as #10, 1:1 ratio)
=________
12. Grams of calcium chloride present in beaker#1 (convert #11 into grams) [#11x111.0 g/mol]
=______________g
13. Percent of calcium chloride=(Grams of calcium chloride/Grams of unknown mixture) x 100=
= [(#12/#3) x100] =_________%
Show all your work and use proper units and significant figures!
1. Calculate the gram molar mass of the following chemicals using a periodic chart:
a. CaCl2 ----------------------------------------------------------------------------------------------------------------grams/mol
b. Na2CO3 ------------------------------------------------------------------------------------------------------------------------grams/mol
c. CaCO3
------------------------------------------------------------------------------------------------------------------------grams/mol
2. A 1.249 g mixture containing calcium chloride and sodium chloride reacts with excess
sodium carbonate as per equation…(i) to precipitate 0.567 g of calcium carbonate.
a. Calculate the moles of calcium carbonate precipitated?
b. How many moles of calcium chloride are present in the mixture?
c.
Convert moles of calcium chloride into grams of calcium chloride.
d. Calculate % calcium chloride in the mixture.
3. Define the following terms:
a. Precipitate
b. Solubility
c. Solute
d. Supernate