DETERMINATION OF EMPIRICAL
Chemistry is based largely on the Law of Definite Composition: Each compound is
characterized by a constant and definite composition. If we know this composition, we
can determine an empirical formula for the compound. In this exercise, if you are
careful, you can determine the empirical formula for one of the compounds of copper
1. Knowledge of the mole concept
2. An understanding of the meaning of formulas
3. The ability to determine the empirical formula of a compound from a knowledge of
the composition. If you do not remember how, go back to your lecture notes and look
up the four steps.
4. As required In GENERAL INFORMATION - LAB, be ready to turn in a COPY of your
5. Be ready to answer the following questions:
A. How long should the beaker of copper and nitric acid be left in the hood?
B. How do you know if you have added enough 1.5M NaOH or KOH?
C. What is a "slurry"?
D. describe the difference between an evaporating dish (which you are to use
today) and a watch glass (which you are not to use today).
E. When should you use purified water and when should you use tap water?
When you finish with this lab you should be able to:
1. Prepare an oxide of copper.
2. Use careful laboratory technique to determine the composition of the oxide.
3. Determine the empirical formula of the oxide based on its composition.
IV. Laboratory Exercise
Oxides are compounds of oxygen and some other element. There are a number of
ways to prepare an oxide of a metal. The metal can be heated in air, or a hydroxide or a
carbonate of the metal can be heated strongly. Heating in air has the disadvantage that
the oxide coating which forms may protect the metal from further reaction, so the
reaction with oxygen may be incomplete. In this exercise, we will prepare an oxide of
copper from the hydroxide, since the reaction is quick and easy.
We will take a known amount of copper metal, allow it to react with nitric acid to form
the nitrate, and then form the hydroxide by adding sodium hydroxide. The hydroxide is
easily converted to the oxide by heating.
Since the oxide contains only copper and oxygen, the difference between the mass of
the dried oxide and the copper that you started with is the mass of oxygen in the
After you have determined the gm of copper and gm of oxygen, you can
calculate the moles of each, and the molar ratio of Cu:O. You can then write
the empirical formula (just like you did in lecture).
The results of a quantitative procedure like this are determined by a lot of
things which can be done right or wrong. Watch for things (other than careless
weighing techniques or spilling something) which can have an effect on your
Weigh out about 4 gm of copper metal (to the nearest 0.01 gm). If copper "turnings" are
used, be careful -- you can cut yourself on the sharp edges. So cut off what you need
with scissors instead of trying to pull out a wad with your bare hands.
gm copper _____gm
Calculate the number of moles of copper from this mass.
moles copper = _____
If copper turnings are used, wad the copper up into a small ball.
Place the copper into a 250 ml beaker.
In the fume hood, measure out about 20 ml of concentrated nitric acid and pour it into
the beaker with your copper.
Leave the beaker in the fume hood until all of the yellow-brown fumes of nitrogen
dioxide are gone. Make sure that there are no unreacted pieces of copper sticking to
the sides of the beaker.
Then take the beaker back to your bench. Measure out 175 ml of 1.5 M NaOH solution.
Slowly and with constant stirring, add small amounts of 1.5 M NaOH solution until you
have added all of the solution. The hydroxide converts the copper nitrate into copper
hydroxide (blue and insoluble), which may be converted by the heat of the reaction into
copper oxide (black and insoluble).
Moisten the end of your stirring rod with the liquid part of the resultant mixture and
touch it to a strip of Red Litmus Paper. If the drop does not produce a bluish-purple wet
spot, then add and mix 5 ml increments of 1.5 M NaOH until you do get a blue spot.
DON'T BE FOOLED BY THE BLUE SOLID THAT YOU MAY TRANSFER TO THE
Place the beaker onto a wire gauze on a ring stand or tripod. Heat with a low flame and
WITH CONSTANT STIRRING (or else it may boil suddenly and everything will be
thrown out of the beaker and all over you!!) until the precipitate has turned completely
Attach the outlet of the vacuum flask to the side-arm of the aspirator on the faucet with
a piece of vacuum tubing.
Place a 7 cm piece of filter paper into the Buchner funnel and place the funnel on the
Turn on the faucet (ALL THE WAY) so that the aspirator forms a vacuum.
Moisten the filter paper with a little purified water.
NOTE: "QUANTITATIVE TRANSFER" IS VERY IMPORTANT IN THIS EXERCISE.
QUANTITATIVE TRANSFER MEANS TO MAKE SURE THAT ALL MATERIAL IS
REMOVED FROM ONE CONTAINER AND PLACED INTO THE OTHER.
Pour the slurry (the soupy mixture of solid and liquid) into the funnel. Wash any solid
still in the beaker into the funnel with water from your wash bottle. Make sure that you
have made a "quantitative transfer".
When most of the liquid has been pulled through, just BEFORE the drying cake begins
to crack much, pour about 10 ml of purified water over the surface of the solid to wash
Repeat with a second 10 ml of purified water when most of the water has been pulled
through, just before the drying cake begins to crack much.
When the solid has dried down to a fairly firm cake (not a sticky mud), turn off the
Pour the solution in the vacuum flask into the "Inorganic" container.
Weigh an evaporating dish to the nearest 0.01 gm.
evaporating dish _____gm
Place the cake from the funnel into evaporating dish. Make sure that all the solid is
removed from the funnel and filter paper: a quantitative transfer. Use your spatula if
Place the evaporating dish onto a tripod. Do not use a wire gauze here.
Heat with the Bunsen burner until all of the solid is completely dry and powdery. You
may need to break up the solid with your stirring rod.
When dry, allow the copper oxide to cool for 5 minutes.
Weigh the copper oxide to the nearest 0.01 gm by difference with the weighed
evaporating dish + copper oxide: _____gm
copper oxide: _____gm
Calculate the amount of oxygen in the copper oxide:
gm copper oxide - gm copper = _____gm oxygen
Calculate the moles of oxygen present in the copper oxide.
moles oxygen = _____
Based on the moles of copper (previously calculated) and of oxygen, calculate the
molar ratio and write the empirical formula.
Empirical formula: _____
Scrape the copper oxide from the evaporating dish into the "Inorganic" container.
You MUST show your instructor that you have cleaned your evaporating dish and
emptied your wash bottle before you get your Lab Report form.