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DETERMINATION of the EMPIRICAL FORMULA
One of the fundamental statements of the atomic theory is that elements combine in simple whole number ratios.
This observation gives support to the theory of atoms, since one would expect whole atoms to combine.
Furthermore, it is observed the combining ratio for a given compound is constant regardless of the origin of the pure
substance. This is known as the Law of constant composition. The mass contribution of each atom in a compound
is a function of the number of atoms in the simplest formula and the relative mass of each atom. The mass
contribution is usually referred to as the percent composition of a compound.
The empirical formula represents the smallest whole number ratio of atoms in a compound. The molecular
formula represents the actual number of atoms in a compound.
In this experiment you will determine the empirical formula of a compound consisting of copper and chlorine. You
will add a known mass of zinc metal to a carefully measured volume of a blue solution known to contain a certain
amount of copper chloride. The zinc metal will replace the copper to produce copper metal and zinc(II) chloride.
As the reaction proceeds the blue color of the solution will disappear and a reddish solid of copper metal will
appear. Once the reaction is complete, you will dry and weigh the copper metal as well as the excess zinc. Knowing
the original mass of the compound present in your 25.00 mL sample, you can determine the empirical formula.
In addition, you will be able to calculate the theoretical yield of copper based on the amount of zinc reacted.
Comparing the actual yield of copper recovered to the theoretical yield, you can determine a percent yield of
copper.
PROCEDURE:
1. Using a clean and dry 10.00 mL graduated cylinder, transfer exactly 25.00 mL of the copper chloride solution (2
x 10.00 mL and 1 x 5.00 mL) to a 150 mL beaker. From the reagent bottle, record the mass of copper chloride
in 100.0 mL. Calculate the mass of copper chloride in 25.00 mL.
2. Obtain some Zn and record its mass to the nearest 0.001 g, making sure you have at least 3.5 g of zinc as you
begin the reaction. Tilt the reaction beaker to a 450 angle and using crucible tongs, slide the piece of zinc down
the side of the beaker into the solution. Do not to splash the solution. Continuously stir the solution with a glass
stirring rod so that the copper which forms does not adhere to the Zn. Allow the reaction to continue until the
blue color has disappeared from the solution.
3. Remove the remaining Zn metal from the solution with tongs. Use a wash bottle of deionized water to wash the
copper off the Zn and into the reaction beaker. You may need to use a rubber policeman to scrape copper off the
zinc strip. Dry the Zn on a paper towel and record the mass. Discard the Zn in the “solids” waste bottle.
4. In a 250 mL beaker, heat some water to boiling. This will be used to steam-dry the copper in step 6.
5. Carefully pour the liquid off without losing any of the copper. Discard this liquid. Wash the copper solid by
adding 10 mL of deionized water to the beaker. Thoroughly stir the mixture, allow the copper to settle and then
carefully decant the wash water from the copper into a second beaker. Repeat with a second 10 mL portion of
deionized water. Discard the wash rinses. Transfer the copper to a large white crucible. Use the rubber
policeman to facilitate the transfer. Rinse the copper twice with 10 mL of methanol. Methanol evaporates faster
than water and will aid in the drying process. Discard the methanol in the “liquids” waste bottle.
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6. Place the crucible over the beaker of boiling water and steam-dry. Stir the copper with the rubber policeman
to facilitate the drying process. Heat until the solid appears dry, then heat an additional five minutes. If green
copper oxides appear, cease heating immediately.
7. Cool the crucible and copper on wire gauze for about five minutes. Transfer the dry and cooled copper to a
piece of weighing paper and record its mass to the nearest 0.001 g in your lab notebook – this is your actual
yield of copper. Discard the copper in the “solids” waste bottle.
CALCULATIONS:
Determination of Empirical Formula: From the mass of copper chloride you started with and the mass of copper produced, determine the percentage of copper and the percentage of chloride in the original solution. Determine the empirical formula by converting your grams of copper and chlorine into moles of copper and
chlorine. The mole ratio of copper and chlorine should be rounded up to 1 significant figure on this lab; hence, a
chlorine to copper ratio of 1.53:1 would indicate a formula of CuCl2.
Write a balanced chemical equation for the reaction of zinc with the copper chloride solution to create zinc(II)
chloride and copper metal. Use your empirical formula for the copper chloride in the equation.
Calculate the grams of zinc reacted (i.e. the initial weight of Zn minus the final weight of Zn after the reaction was
complete), then calculate the theoretical yield of copper using the formula:
theoretical yield of Cu = (grams of zinc reacted in the reaction) * 0.9718
Finally, determine the percent yield of copper:
% Yield Cu = (actual yield of Cu in g) / (theoretical yield Cu in g) x 100%
Extra credit (1 point) will be awarded students who successfully explain (using math and words) the origin of
the 0.9718 number used in determining the theoretical yield of copper in their lab report.
POSTLAB QUESTIONS:
1. Which of the reactants was the limiting reagent? Briefly explain your answer. What is the largest source of error
in this experiment?
2. Explain the effect each of the following would have on the experimentally determined %Cu. Would it
increase, decrease or have no effect on the %Cu?
a. Some solution splashed onto the bench when the zinc was plopped into the beaker.
b. The student removed the zinc before the blue color disappeared from the solution.
c. The student did not completely dry the Cu before the final weighing
3. Carlos Chromium reacted 0.672 g of iron with oxygen from the air. The final product weighed 0.961 g.
Determine the empirical formula of the iron oxide product.
4. A 25.000 g sample of calcium reacts with excess chlorine gas to produce 69.224 g of a white solid. Determine
the empirical formula of the resulting calcium chloride compound.
5. Your final mass of zinc was less than your initial. Where did the zinc go? Explain.
6. Determine the %Cl by mass value if the sample was pure copper (II) chloride.