EXPERIMENT Elemental Mass Percent and Empirical Formula From Decomposition 6 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. OBJECTIVE Ring Stand Bunsen Burner Rubber tubing A P P A R A T U S 15 cm Pyrex test tube Glass tubing (bent) APPARATUS AND CHEMICALS C H E M I C A L S Copper oxide (Cu x O y ) Elemental Mass Percent In today s lab, the decomposition of copper oxide (Cu x O y ) 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: 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.
Step 1: Step 2: Step 3: Determine the mass of each element present. Convert the individual elemental masses into the number of moles of each element. 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 H 2 O 1 or simply H 2 O Decomposition of Cu x O y Copper oxide exists as either Copper I (cuprous) or Copper II (cupric) oxide. Heating either of these compounds with methane (CH 4 ) gas will produce copper metal. Your unknown could be copper I oxide or copper II oxide. PROCEDURE 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. CHEM 1001 - Experiment 6 6-2
hold the test tube over the copper oxide container and add a little copper oxide. Rubber tubing to gas outlet 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. Figure 1 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]. CHEM 1001 - Experiment 6 6-3
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 6-4
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 Name: Section: 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 1001 - Experiment 6 6-5