Experiment: Empirical Formula Determination
PURPOSE
To determine the empirical formula of copper oxide.
THEORY
Copper forms two oxides: copper (I) oxide (Cu2O) and copper (II) oxide (CuO). Empirical formula
determination is used to determine one oxide from the other.
MATERIALS
Chemicals/consumable
2g copper oxide, 2g zinc granules, 50 cm3 2M H2SO4
Glassware
250 cm3 beaker, weighing bottle, evaporating basin
Equipment
electronic balance, steam bath, safety glasses.
SAFETY
Wear safety glasses when using sulfuric acid. Sulfuric acid is corrosive
DURATION
2 x 40-50 minutes
PROCEDURE
1. Using a weighing bottle, find the exact mass of approximately 2g of copper oxide.
2. Transfer the copper oxide to a 250 cm3 beaker and add 50 cm3 of 2M sulphuric acid. If necessary, heat
the mixture by standing the 250 cm3 beaker in a larger beaker of hot water. Stir with a glass rod until the
reaction is complete
3. When all the copper oxide has reacted, add 50 cm3 of water and approximately 3g of granulated zinc.
Swirl and allow to react overnight. (If the solution is still blue the next day, add another 1g of zinc and
allow to stand several hours longer).
4. Decant the supernatant liquid and carefully wash the deposited copper with about 50 cm3 of water.
Wash it three more times.
5. Find the mass of a clean, dry evaporating basin.
6. Using the minimum amount of water, transfer the copper from the beaker to the evaporating basin.
7. Dry the copper by heating over a steam bath. When cool, find the mass of the evaporating basin plus
copper.
QUESTIONS
1. Calculate the mass of copper oxide used.
2. Calculate the mass of copper that was in the copper oxide.
3. Calculate the mass of oxygen in the copper oxide.
4. Calculate the amount in moles of copper and oxygen in the copper oxide.
5. Calculate the empirical formula of the compound.
6. How does your result compare with the formulae for copper oxides? Which oxide did you use?
7. What are some of the major sources of error in this experiment?
8. Write a balanced equation for the reaction that occurs between:
(i)
sulphuric acid and copper (II) oxide
(ii)
zinc and the copper sulphate solution;
(iii)
sulphuric acid and the left-over zinc. (The gas evolved in this reaction is hydrogen gas (H2).)