2.
Name:
Partner:
Date:
Period:
Relative Masses of Atoms
___________________
___________________
___________________
____
Prelab Questions
1. What is the purpose of this lab?
_________________________________________________________________________________________
_________________________________________________________________________________________
2. How will you be able to tell when all of the CuO is converted to Cu? (What does Cu look like?)
_________________________________________________________________________________________
_________________________________________________________________________________________
3. What is the biggest danger in this lab? How will you avoid this danger?
_________________________________________________________________________________________
_________________________________________________________________________________________
4. In step 2 of the procedure, it says to leave the CuO confined to the bottom of the test tube.
Explain why it is important for all of the CuO to be at the bottom of the test tube and not on the
sides.
_________________________________________________________________________________________
________________________________________________________________________________________
5. Describe the way in which you will determine if the test tube is too hot to handle once the reaction
is complete.
_________________________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
Relative Masses of Atoms
INTRODUCTION:
CP1~1213
In this experiment you will convert a sample of pure copper(II) oxide, CuO, to metallic copper, Cu, by
reacting it with the gas used fuel your Bunsen burner. If the gas is assumed to be methane, CH4, the reaction
may be expressed this way:solid copper(II) oxide reacts with methane gas to produce solid copper, carbon
dioxide, and water vapor
4 CuO(s) + CH4(g)  4 Cu(s) + CO2(g) + 2 H2O(g)
Since all products except the copper escape from the system as gases, the amount of metallic copper
produced can be easily determined. The mass ratio of copper to oxygen can then be determined from this
reaction. This can be used to determine the relative mass of copper if the accepted relative mass of oxygen is
known.
PROCEDURE:
1. Determine the mass of a clean, dry 18 x 150 mm (i.e., large) test tube to the nearest 0.01 g.
2. Roll a piece of paper into a tight cylinder and place it in the previously
weighed test tube. Transfer approximately 2.0 g of CuO to the bottom
of the test tube through the paper cylinder. Carefully remove the
paper cylinder, leaving the CuO confined to the bottom of the test
tube. Determine the mass of the test tube containing the CuO sample
to the nearest 0.01 g.
3. Complete the apparatus setup ~ Be sure to wear safety goggles
and keep hair tied back and loose clothing away!!
4. Have your teacher help you light and adjust the flame of a Bunsen
burner to obtain the hottest possible flame. Your teacher will also help
you light the gas coming from the open end of the test tube. Heat
the CuO by placing the burner under the end of the test tube.
Continue to heat the copper(II) oxide until it is fully converted to
metallic copper. (How can you determine this?) You may be told to
sit down for while the reaction takes place. It usually takes about 5
minutes if you have a hot burner flame, but can sometimes take
much longer. Once the reaction is complete, carefully heat the wall of the test tube to “chase out” any
condensed moisture, which is another product of this reaction.
5. Turn off the Bunsen burner, but allow the gas to continue to flow through the glass tube and to burn at
the mouth of the test tube. Wait 5 minutes, or until the sealed end of the test tube is cool to the touch.
When cool, turn off the gas flowing to the tube. There should no longer be a flame at the mouth,
extinguish it if necessary. Allow the open end of the test tube to cool.
6. Carefully remove the cooled test tube from your equipment assembly and determine its mass with the
copper inside.
7. Shake the solid copper plug from the test tube and rub it firmly on a hard surface or hammer it. Can you
observe the characteristic metallic luster of the copper? Is it malleable? Your sample is free to take home.
Clean your lab area but leave the ring stands, clamps, test tube, glass tube and Bunsen burner set up for
the next lab group.
8. Complete your calculations, exchange class data, and finish your lab report.
DATA:
Mass of empty
test tube
Mass of test
tube + CuO
Mass of test
tube + Cu
Calculated mass
of CuO
Calculated mass
of Cu
Calculated mass
of O in original
CuO sample
CALCULATIONS:
1. Calculate the mass ratio,
Cu
, from your data, expressing your answer as a decimal fraction.
O
mCu
mass of copper
=
=
mO
mass of oxygen
2. Compare your
=
1
Cu
mass ratio with that of other class members. This mass ratio will be important in your
O
study of the relative masses of atoms.
Class Results:
Group
Ratio
Group
Ratio
Using the periodic table, calculate the theoretical Cu/O mass ratio.
3. Based on your results (answer from #1) and the accepted value of 15.9997 for the relative mass of
oxygen, calculate the relative mass of copper. (look at the formula used to calculate answer in #1)
mass of copper mCu
=
mass of oxygen mO
4. Calculate your % error for the calculated relative atomic mass of copper.
Questions:
1. What is the Law of Conservation of Mass and how does it apply to this lab?
2. Mercury(II) oxide is a compound composed of the elements mercury and oxygen. An 8.25-g sample of this
compound is carefully heated, decomposing the compound into elemental mercury liquid and oxygen gas.
[Caution: Do not attempt to carry out this experiment; mercury is extremely toxic.] At the end of the
reaction, 7.64 g of pure mercury are found.
a. From these data, calculate the mass ratio,
Hg
, for mercury(II) oxide.
O
b. Write the equation for this decomposition reaction.
3. Explain the effect on the
Cu
of the following (does it increase, decrease or stay the same):
O
a. Oxygen not fully driven off
b. Copper was removed from the test tube before the final mass was determined.
c. The test tube was wet at the beginning of the lab
d. You turned off the gas at the mouth of the test tube before the copper cooled.
4. What is the Law of Definite Proportions and how does it apply to this lab?