General Chemistry: % Composition Lab
Name______________________
Some compounds exist in nature as hydrated compounds. That means that the crystal form of
the compound includes in its structure some water molecules. The formula for such a hydrated
compound will always include the number of water molecules. Some examples are:
Sodium carbonate Na2CO3 .10H20
Copper(II)sulfate
CuSO4.5H20
In figuring out the Gram Formula Mass (GFM) of hydrated compounds, you must
include the water molecules in the amount of grams.
Hydrated copper(II)sulfate (CuSO4.5H2O) is a blue crystalline material. If it is heated the water will
evaporate and it will become anhydrous copper(II)sulfate (CuSO4).
Hydrated means the crystal has water as part of the formula.
Anhydrous means the water has been removed. (without water)
Equipment:
bunsen burner
250ml beaker
beaker tongs
balance
gauze pad
watch glass
ring stand
iron ring
SAFETY GOGGLES
Procedure: *Safety goggles must be worn throughout the entire lab until everything is
cleaned up and put away. If you take them off you will be asked to sit
down and receive a zero for the lab.
1. Find the mass of the beaker. Record mass in data table to the nearest hundredths place.
2. Place one small teaspoon of the hydrated copper(II)sulfate in the beaker.
3. Find the mass of the beaker + hydrated copper(II)sulfate. Record in data table.
4. Determine the mass of the hydrated copper(II)sulfate.
5. Set up the ring stand by attaching the iron ring to the ring stand and placing the gauze pad on
top the iron ring. Make sure the Bunsen burner can fit easily under the iron ring leaving ~3” in
between.
6. Place the beaker on top of the wire gauze and place the watch glass on top of the beaker.
7. Heat gently for two minutes and record your observations.
8. Remove the watch glass using the beaker tongs. (CAUTION HOT!)
9. Continue heating for ~10min. until the color completely changes and is no longer blue.
10. Turn off the burner and let the anhydrous sample cool for 5min.
11. Using beaker tongs transfer the beaker to the balance and find the mass of the beaker +
anhydrous sample. (CAUTION HOT!) Record in data table.
12. Determine the mass of the anhydrous sample. Record in data table.
13. CLEAN UP WORK STATION.
OBSERVATIONS: ___________________________________________________________
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DATA TABLE:
Mass of beaker
Mass of beaker and hydrated CuSO4
Mass of hydrated CuSO4
Mass of beaker and anhydrous CuSO4
Mass of anhydrous CuSO4
Mass of H2O
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CALCULATIONS: (SHOW ALL WORK!!)
1. Determine the % of water in your hydrated sample.
% H2O =
mass H2O
mass hydrated CuSO4
2. Determine the % CuSO4 in your hydrated sample.
%CuSO4 = mass of anhydrous CuSO4
mass of hydrated CuSO4
3. Determine the % H2O using (GFM) Gram Formula Mass.
% H2O =
5(GFM H2O)
GFM (CuSO4 + 5H2O)
4. Determine the % CuSO4 using (GFM) Gram Formula Mass.
%CuSO4 =
GFM CuSO4
GFM (CuSO4 + 5H2O)
5. Determine the error for the % H2O.
% H2O error = /lab % H2O – GFM % H2O/
6. Determine the error for the % CuSO4.
% CuSO4 error = /lab %CuSO4 – GFM % CuSO4/
10. List three sources of possible error. (Why there are differences between your lab %’s and the GFM %’s)