Experiment 7: The Formula of Copper Oxide
The formula of a chemical compound gives the ratios of all atoms in the compound. One way to determine the formula of a compound is to break it down into its constituent elements and determine the relative masses and numbers of atoms.
In this lab you will use methane gas to reduce the compound copper oxide into copper metal and oxygen gas.
You will determine the mass of copper and oxygen in the compound. From this you can determine the percentages of each element and the formula of the compound.
Procedure
1.
Obtain a test tube and rubber hose assembly. Measure and record the mass of the empty test tube to the nearest 0.01 gram.
2.
Measure out about 2.00 grams of copper oxide powder. Place this in the test tube using a slip of paper as a funnel as explained your teacher. Try to get all the copper oxide into the bottom.
3.
Measure and record the mass of the test tube with the copper oxide
4.
Clamp the test tube to a ring stand. Be sure to clamp near the mouth of the test tube. With another clamp set up the house assembly as instructed by you teacher. Attach the hose to one of the gas jets.
Make sure the glass tubing is close to the powder.
5.
Slowly turn on the gas and light the excess gas that comes out of the mouth of the test tube carefully!
6.
Now light your Bunsen burner and adjust the flame to a hot flame. Heat the copper oxide powder until it glows red hot. Keep heating for about 20 minutes. Note and record the changes that occur. You may have to move the burner around to heat the powder evenly on all sides.
7.
After about 20 minutes of heating, turn off the burner but leave the gas on for another two minutes.
Why?
8.
Turn off the gas and let the test tube cool. Then measure the mass of the test tube and its contents.
9.
Tap the test tube carefully to remove the contents. Rub it on your lab bench. Describe the properties of the contents.
10.
Return the equipment
Calculations and Questions
1.
From your mass measurements determine the following: a.
The exact mass of copper oxide powder used b.
The mass of copper produced at the end c.
The mass of oxygen that must have been in the powder
2.
Find the percentage of oxygen in the copper oxide powder. Determine the percentage of copper in the compound.
3.
Divide the grams of oxygen in the compound by the mass of one oxygen atom as found on your periodic table. This gives the number of moles of oxygen in the compound.
4.
Divide the grams of copper in the compound by the mass of one copper atom as found on your periodic table. This gives the number of moles of copper in the compound.
5.
Find the ratio of copper atoms to oxygen atoms by dividing the number of moles of each. Round this off to the nearest whole number. From this ratio what do you think the formula for this compound of copper oxide? What are the two possible formulas of copper oxide?
6.
Divide the mass of copper in your compound by the mass of oxygen and round off to a whole number.
This tells us how many times heaver a copper atom is than an oxygen atom. Does this agree with the relative masses on the periodic table? (Divide the mass of copper atom by the mass of an oxygen atom from the periodic table.)
7.
After heating the copper oxide why did you leave on the gas as the product cooled?
8.
Was this a chemical change? Give evidence for your answer.
Write a formal conclusion.