Name:_________________________________
Lab Day: M T W Th F
Date:_____________________
CH105
(circle one)
Pre-Lab 5: Quantitative Analysis with Decomposition Reactions
Directions: Read the Goals, Background, Safety, and Procedure sections for this
experiment, then answer the following questions in the space provided. For
calculation questions, show all work and report answers in a box with the appropriate
significant figures and units. Pencil is acceptable for this assignment.
Automotive air bags inflate when sodium azide, NaN3, rapidly decomposes into its
component elements.
2 NaN3 (s) → 2 Na (s) + 3 N2 (g)
1. Write the equation in words.
2. Write the mole relationship between sodium azide and nitrogen gas as an equality
as well as two conversion factors.
3. How many moles of sodium azide are in a 2.8 g sample?
4. How many moles of nitrogen gas will be produced if 2.8 g NaN3 react?
5. What mass of nitrogen gas will be produced if 2.8 g NaN3 react?
6. What volume of nitrogen gas will be produced if 2.8 g NaN3 react? Assume the gas
has a density of 1.25 g/L.
C.I.Martinez
1
PCC-RC
Experiment 5:
Quantitative Analysis with Decomposition Reactions
Goals:  Use heat to the remove the waters of hydration from a hydrate.
 Use heat to decompose an oxygen containing compound.
 Calculate the number of waters of hydration for barium chloride.
 Calculate the percentage of KClO3 in a mixture of KClO3 and KCl.
Background:
A hydrate is a solid substance that contains water molecules as part of their crystal
structure. Hydrates are usually salts and always contain a specific number of
molecules of water per formula unit of the solid. The water molecules are weakly
bound to the ions in the crystal lattice. They are called waters of hydration.
Iron (III) chloride hexahydrate (FeCl3 6H2O) is a hydrate. For every one formula unit
of FeCl3, six waters of hydration are coordinated to the solid. The water that is
present in these salts is usually bound to the cations in the salt. In the iron (III)
chloride hexahydrate, the water is associated with the Fe 3+ ion as shown below.
H 2O
H 2O
H 2O
H 2O
Fe3+
H 2O
H 2O
2-
Sometimes the waters of hydration are coordinated to the anion (like SO 4 ) by
hydrogen bonds. This happens in copper (II) sulfate pentahydrate
(CuSO4 5H2O), in which four of the water molecules are coordinated to the Cu 2+
2cation and the other water molecule is coordinated to the SO4 via hydrogen bonds.
The waters of hydration can be removed by heating the hydrate to temperature slightly
above the boiling point of water. The resulting solid is an anhydrous (without water)
salt. The equation for this decomposition reaction looks like:
FeCl3 6H2O (s)  FeCl3(s) + 6H2O (g)
In the dehydration reaction, the crystal structure of the solid will change and the
appearance of the salt may also change. On heating, the color and consistency of the
crystals may change.
Hydrates that lose water upon standing in a dry environment are said to be
efflorescent. The amount of water lost depends upon the amount of water in the air,
as measured by relative humidity. Cobalt chloride is a classic example of an
efflorescent material. In moist, warm air, it is fully hydrated and is red; in dry, cold
air it loses most of its waters of hydration and is blue; and at intermediate humidities,
it exists as a violet dihydrate.
C.I.Martinez
2
PCC-RC
Quantitative Analysis with Decomposition Reactions  3
Hydrates that absorb water from the air or other sources so strongly that they can be
used to dry liquids or gases are said to be hygroscopic. When the hygroscopic
materials are used to dry liquids or gases, they are called desiccants.
In this experiment, you will determine the coefficient for water in a hydrate by
weighing the hydrate, heating the hydrate, and reweighing the anhydrous compound.
The difference in weight will be the water driven off.
Some crystalline solids do not contain water of hydration but will still undergo
decomposition reactions when heated. When one of the products of decomposition is
a gas, the change in mass from the gas given off can be used in much the same way
as the mass of water given off from a dehydrated hydrate. The mass of gas produced
and the equation for the decomposition reaction can be used to calculate the mass of
solid that was decomposed as well as the percentage of the solid present in a mixture
that contains other solids which do not decompose on heating.
Solid potassium chlorate (KClO3) decomposes upon heating to solid potassium
chloride (KCl) and oxygen gas (O2).
In words:
heating
 potassium chloride + oxygen gas
potassium chlorate 
Balanced equation:

2 KClO3 (s) 
2 KCl (s) + 3 O2 (g)
Based on the equation, we see that for every two moles of potassium chlorate that
decompose, two moles of potassium chloride and three moles of oxygen are produced.
Based on this balanced equation, several conversion factors can be written for use in
dimensional analysis.
2 moles KClO3 = 2 moles KCl

2 moles KClO3
2 moles KCl
and
2 moles KCl
2 moles KClO3
or
or
1 mole KClO3 = 1 mole KCl

1 mole KClO3
1 mole KCl
and
2 mole KClO3 = 3 moles O2

2 moles KClO3
3 moles O2
and
3 moles O2
2 moles KClO3
2 moles KCl = 3 moles O2

2 moles KCl
3 moles O2
and
3 moles O2
2 moles KCl
1 mole KCl
1 mole KClO3
The oxygen produced in this reaction escapes into the air. So, the mass of oxygen
produced in this reaction can be calculated by subtracting the mass after heating from
the mass before heating.
Safety:
Hot glassware looks like cold glassware. Avoid contact with hot materials.
C.I.Martinez
3
PCC-RC
4  Experiment 5
Procedure:
Part 1. Determination of the formula of a hydrate.
1. Clean a porcelain evaporating dish with soap and water. Permanent stains that
will not come off can be ignored, but clean all removable material. Rinse the
evaporating dish with DI water.
2. Place the evaporating dish on a piece of wire gauze suspended by an iron ring on a
ring stand.
3. Heat the evaporating dish gently for 1-2 minutes (with a yellowish flame) and make
sure no soot forms on the bottom of the dish. Then, heat strongly with the hottest
part of a blue flame for another 5 minutes.
4. Turn off your flame, then use tongs to carefully remove the evaporating dish from
the ring stand and place it on a cool piece of wire gauze. Allow the evaporating
dish to cool for a minimum of 10 minutes.
5. After the evaporating dish is cool, take its mass on a balance.
6. Add 0.4-0.5 g of barium chloride hydrate to the evaporating dish. Record the exact
mass of barium chloride hydrate added on your data sheet.
7. Place the evaporating dish and barium chloride hydrate on the piece of wire gauze
suspended by an iron ring on a ring stand.
8. Heat the evaporating dish with moderate heat (light blue flame) from the Bunsen
burner for 2-3 minutes, then heat it strongly for 5 minutes. Watch for spattering
solid. We do not want to lose any solid from popping out of the container.
9. After the heating is completed, place a watch glass over the evaporating dish so
that it covers the evaporating dish completely. Allow the evaporating dish and its
contents to cool for a minimum of 10 minutes.
10. Reweigh the cool evaporating dish and its contents. (Be sure to remove the watch
glass.) Record the mass on your data sheet.
Part 2. Determination of Percentage KClO3 in an unknown KClO3/KCl mixture.
1. Weigh a clean, dry 10 cm test tube on an electronic balance. Record the mass on
your data sheet.
2. Add a 0.4-0.5 g sample of the unknown mixture containing KClO3 and KCl to your
test tube. Record your unknown letter on your data sheet.
3. Record the mass of your sample and test tube on your data sheet.
C.I.Martinez
4
PCC-RC
Quantitative Analysis with Decomposition Reactions  5
4. Mount the test tube with the unknown sample on a ring stand with a clamp.
Rotate the clamp so the test tube is not vertical but is tilted at about a 45° angle.
BE SURE THE TEST TUBE IS NOT POINTED AT YOU OR ANYONE ELSE.
5. Use a moderate flame (light blue) to heat the test tube gently. The solid will be
agitated and might melt as the decomposition reaction takes place. Control the
heating carefully so that the reaction does not expel reactants from the mouth of
the test tube.
6. After the reaction appears to be completed (agitation stops and mixture looks
solid), increase the intensity of the heating and heat strongly for 2 minutes.
7. After the heating is complete, allow the test tube and its contents to cool for a
minimum of 15 minutes. Then, weight the test tube and contents, and record the
mass on your data sheet.
8. Rinse the heated sample down the sink with lots of water.
C.I.Martinez
5
PCC-RC
Name:_________________________________
Lab Day: M T W Th F
Date:_____________________
CH105
(circle one)
Data Sheet 5: Quantitative Analysis with Decomposition Reactions
Directions: Record the data as it is collected onto this sheet in BLUE or BLACK ink.
Do not use white out. Correct mistakes by making a single line through the error and
writing the new information above or beside the mistake.
Part 1.
Mass of evaporating dish (clean, heated, cooled) (g)
____________
Mass of BaCl2xH2O and evaporating dish (g)
____________
Mass of BaCl2xH2O (g)
____________
Mass of heated anhydrous BaCl2 and evaporating dish (g)
____________
Mass of anhydrous BaCl2 (g)
____________
Moles of anhydrous BaCl2
____________
Mass of water in BaCl2xH2O (g)
____________
Moles of water
____________
Moles ratio of BaCl2 to H2O
____________
Value of x in BaCl2xH2O
____________
Formula: BaCl2xH2O with x plugged in
____________
C.I.Martinez
6
PCC-RC
Experiment 5
Part 2.
Data Sheet
Unknown letter
____________
Mass of test tube (g)
____________
Mass of KClO3/KCl mixture + test tube (g)
____________
Mass of KClO3/KCl mixture (g)
____________
Mass of heated KClO3/KCl mixture + test tube (g)
____________
Mass of heated KClO3/KCl mixture (g)
____________
Mass of O2 lost from the KClO3/KCl mixture (g)
____________
Moles of O2 lost from the KClO3/KCl mixture
____________
*Moles of KClO3 converted to KCl
____________
Mass of KClO3 in the original KClO3/KCl mixture (g)
____________
Mass percent of KClO3 in the original KClO3/KCl mixture (g)
____________
*Use “Moles of O2 lost from the KClO3/KCl mixture” and the relationship between moles
of O2 and moles of KClO3 from the balanced equation to find the “Moles of KClO3
converted to KCl.”
C.I.Martinez
7
PCC-RC
Name:_________________________________
Lab Day: M T W Th F
Date:_____________________
CH105
(circle one)
Post Lab 5: Quantitative Analysis with Decomposition Reactions
Directions: Answer the following questions in the space provided. For short answer questions, write
complete sentences and provide a reason for the answer. For calculation questions, show all work and
report answers in a box with the appropriate significant figures and units. Pencil is acceptable for this
assignment.
1. Write an equation for the decomposition of copper sulfate pentahydrate.
Solid calcium carbonate (CaCO3) decomposes on heating into a solid, calcium oxide (CaO),
and a gas, carbon dioxide (CO2).
Balanced equation:

CaCO3 (s) 
CaO (s) + CO2 (g)
A student had a 0.500 g sample of CaCO3. After heating the sample, the student reweighed
the sample and found the mass after heating was 0.380 g.
2. How many moles of CaCO3 were in the original sample?_________________________________
Show work.
3. How many moles of CO2 could theoretically evolve from the original sample?______________
Show work.
4. How many grams of CO2 actually evolved from the heated sample?_______________________
Show work.
5. How many moles of CO2 actually evolved from the heated sample?_______________________
Show work.
6. Is the reaction complete? Has all of the CaCO3 decomposed into CO2 and CaO?__________
Explain.
C.I.Martinez
8
PCC-RC