Copyright 2005 Chem21 LLC. No part of this work may be reproduced, transcribed, or used

in any form by any means – graphic, electronic, or mechanical, including, but not limited to,

photocopying, recording, taping, Web distribution, or information storage or retrieval systems

– without the prior written permission of the publisher. For permission to use material from

this work, contact us at info@chemistrylabmanual.com. Printed in United States of America.

EXPERIMENT

6

OBJECTIVE

APPARATUS

AND

CHEMICALS

Elemental Mass Percent and

Empirical Formula From

Decomposition of a Copper Oxide

Prepared by Edward L. Brown, Lee University and

Verrill M. Norwood, Cleveland State Community College

The student will heat copper oxide in a methane atmosphere forming elemental

copper. The masses of the original CuO and the resulting Cu will be used to

arrive at the mass percent and empirical formula of the copper oxide.

Elemental Mass Percent

In today’s lab, the decomposition of copper oxide (CuxOy) provides the mass data needed

to determine the mass percent of both elements.

Empirical Formulas

The empirical formula of a compound is the simplest whole number ratio of its elements.

This ratio can be either an atomic ratio or a molar ratio. The experimental determination

of a compound’s empirical formula occurs in three steps:

AA PP PP AA RR AA TT UU SS

Ring Stand 15 cm Pyrex test tube

Bunsen Burner Glass tubing (bent)

Rubber tubing

CC HH EE MM II CC AA LL SS

Copper oxide (CuxOy)

Elemental Mass Percent and Empirical Formula From Decomposition

of a Copper Oxide

CHEM 1001 - Experiment 6 6-2

Step 1: Determine the mass of each element present.

Step 2: Convert the individual elemental masses into the number of moles of

each element.

Step 3: Express the molar ratio of the two elements by dividing the number of

moles of each element by the smaller of the two numbers. This will

assign “1 mole” to the element with the smallest number of moles and

the other element will be assigned a number greater than or equal to “1

mole”.

Sample Problem 1: The electrolysis (decomposition) of water produces hydrogen and

oxygen gases. From the mass of the elements given in step 1, what is the empirical

formula of water?

Step 1: Mass of the Elements 0.002085 g H 0.01715 g O

Step 2: Moles of the Elements 2.085E-3 g ÷ 1.0079 g/mol 1.715E-2g ÷ 16 g/mol

2.0686E-3 mol H 1.072E-3 mol O

Step 3: Divide by smallest moles 1.93 mol H 1.00 mol O

Step 4: Write the empirical formula H2O1 or simply H2O

Decomposition of CuxOy

Copper oxide exists as either Copper I (cuprous) or Copper II (cupric) oxide. Heating

either of these compounds with methane (CH4) gas will produce copper metal. Your

unknown could be copper I oxide or copper II oxide.

1. Flame dry a Pyrex test tube (15 cm or longer) by holding it vertically (mouth upward)

with tongs in the flame of a Bunsen burner for 3 – 4 minutes. Rest the hot test tube on

a clean dry surface until it has cooled to room temperature (about 5 minutes).

Determine the mass of the test tube [Data Sheet Q1] by taring a small beaker on the

balance and then placing your test tube in the beaker.

2. After recording the mass of the test tube, add 1.000 g 0.200 g of copper oxide.

PROCEDURE

Elemental Mass Percent and Empirical Formula From Decomposition

of a Copper Oxide

CHEM 1001 - Experiment 6 6-3

hold the test tube over the copper

oxide container and add a little

copper oxide.

gently place the test tube in the

beaker and note the mass.

if the mass is 1.000 g 0.200 g

greater than the mass of the empty

test tube, record the exact mass

[Data Sheet Q2], if not, add more

copper oxide.

Record the 4 digit Unknown

Number of the Copper Oxide you

used [Data Sheet Q3].

3. Clamp the test tube to a ring stand with a

bare metal test tube clamp.

4. Assemble the remaining apparatus as shown in Figure 1 but DO NOT TURN ON

THE GAS.

5. Next, light only the Bunsen burner – THIS SHOULD NOT BE AN EXTREMELY HOT

FLAME.

6. HAVE YOUR INSTRUCTOR ASSIST YOU WITH THE

LIGHTING OF THE GAS THAT EXITS THE TEST TUBE.

7. Heat the copper oxide with a cool flame for 5 minutes.

8. Then, adjust the Bunsen burner to produce a hot flame. Heat the test tube with this hot

flame for 10 – 40 minutes (depending on the type of copper oxide used). You will see

a copper colored compound form during this heating process. Periodically ask your

Instructor if your experiment is finished or not.

9. If your Instructor says that your experiment is finished, turn off the gas to the Bunsen

burner, but allow the flame to keep burning at the mouth of the test tube while the rest

of the test tube cools down.

10. After 5 minutes, turn off the gas to the test tube and carefully remove the glass tubing

(Caution: HOT!). Allow the test tube to cool to room temperature.

11. Record the mass of the test tube and copper metal [Data Sheet Q4].

Figure 1

Rubber tubingto gas outlet

Elemental Mass Percent and Empirical Formula From Decomposition

of a Copper Oxide

CHEM 1001 - Experiment 6 6-4

12. Remove the copper from the test tube and rub it gently on a hard surface (i.e. the

bottom of a porcelain well plate) to produce the familiar shiny look and color of

copper. You may dispose of this copper metal in the trash OR keep it as a lab

souvenir.

Waste Disposal: If you don’t keep it as a souvenir, the elemental copper can be

placed in the regular trash container.

Lab Report: Once you have turned in your Instructor Data Sheet, lab attendance will

be entered and you will be permitted to access the online data submission / calculation

part of the lab report (click on Lab 6 – Empirical Formula of Copper Oxide). Enter

your data accurately to avoid penalty. The lab program will take you in order to each

calculation. If there is an error, you will be given additional submissions (the number and

penalty to be determined by your instructor) to correct your calculation.

CHEM 1001 - Experiment 6 6-5

CHEM 1010 - Lab 6 Student Data Sheet

1. Mass of the test tube g

2. Mass of the test tube + Copper Oxide g

3. Unknown Number of the Copper Oxide (4 digits)

4. Mass of the test tube + Copper metal g

CHEM 1010 - Lab 6 Instructor Data Sheet

1. Mass of the test tube g

2. Mass of the test tube + Copper Oxide g

3. Unknown Number of the Copper Oxide (4 digits)

4. Mass of the test tube + Copper metal g

Name:

Section: