Elemental Mass Percent and Empirical Formula from Decomposition

Transcription

1 EXPERIMENT Elemental Mass Percent and Empirical Formula from Decomposition 10 Prepared by Edward L. Brown, Lee University 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 Compounds containing stable elements can be assayed to determine their elemental composition. A host of companies provide these elemental analysis reports to academic and industrial clients. The techniques and instruments used in these companies include Neutron Activation Analysis (NAA), Inductively Coupled Plasma Mass Spectroscopy (ICP-MS), and Proton Induced X-Ray Emission (PIXE), to name a few. At the introductory level of elemental analysis, we will limit our focus to compounds containing two (at most three) elements. In today s lab, the decomposition of Copper oxide provides the mass data needed to determine the mass percent of both elements. 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.

2 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: 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. This sequence of steps can be summarized using a table similar to Table 1. Sample Problem 1: The electrolysis (decomposition) of water produced 23.2 ml of Hydrogen gas (density = g/l) and 12 ml of Oxygen gas (density = g/l). What is the empirical formula of water? Step 1: Mass of the Elements g H g O Step 2: Moles of the Elements 2.085E-3 g g/mol 1.715E-2g 16 g/mol E-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 Percent Composition from Empirical Formulas The percent composition of a compound is easily calculated if its empirical formula is known. An empirical formula gives us two essential facts about a compound s elemental composition: the identity of the elements and their relative ratios. For example, water (H 2 O) is composed of 2 atoms of hydrogen and 1 atom of oxygen. This information, Experiment

3 coupled with information displayed in the Periodic Table, allows the determination of the percent hydrogen and oxygen in water. Sample Problem 2: Determine the percent hydrogen and oxygen in water. Step 1: Moles of the Elements 2 mol H 1 mol O Step 2: Mass of the Elements 2 mol H1.008g/mol 1 mol O 16 g/mol g H 16 g O Step 3: Add the masses together to find the total mass g H 2 O Step 4: Divide the elemental masses by the total masses and multiply by 100 (2.016 / )100 (16 / ) % H % O Obviously, the reversal of these four steps will give the empirical formula from the percent composition. It should be noted that the mass percent of hydrogen and oxygen in water will always be % and % regardless of the source of the water. This observation led to the LAW OF DEFINITE PROPORTIONS. Molecular Formulas from Empirical Formulas While the empirical formula by definition shows the smallest whole number ratio of the elements, it may or may not be the molecular formula. The molecular formula reveals the actual elemental composition in a distinct molecule. For example, the molecular formula of hydrogen peroxide is H 2 O 2 but the smallest whole number ratio of the elements is HO (its empirical formula). We can obtain the molecular formula from the empirical formula if the molecular weight of the compound is known. It turns out that the molecular formula is the same whole number multiple of the empirical formula as the molecular mass is of the empirical mass. Comparing H 2 O 2 (molecular formula) to HO (empirical formula) Experiment

4 shows that the whole number multiple is 2. Two times HO will give H 2 O 2. In addition, the empirical mass of HO is g/mol ( ) while the molecular mass of H 2 O 2 is g/mol (( )+( )) again a multiple of 2. A template has been developed to guide you through the calculations in a problem of this type. This template is a combination of the approaches used previously in Sample Problem 1 and 2. After reading a problem, place the starting information into this template and fill in all missing information between your starting point and the answer. Sample Problem 3: The organic compound n-octylacetate belongs to a class of generally pleasant smelling compounds called esters. In fact, n-octylacetate smells like oranges. If this ester contains % C, % H, and % O by mass, what is its molecular formula? The molar mass of n-octylacetate is g/mol. Element Symbol C H O Percent (%) Mass (g) (if % is given, assume 100 g) Elemental Molar Mass (g/mole) (report to 1 SF > than starting number) Moles Empirical Molar (or Atomic) Ratio (whole numbers only) Empirical Formula C 5 H 10 O Empirical Mass (g/mole) Molecular Mass (g/mole) Molecular Molar Ratio Molecular Formula C 10 H 20 O 2 Table 1 Experiment

5 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. Both of these compounds have valuable chemical applications: cuprous oxide is used as a fungicide and an insecticide primarily in the fruit industry, cupric oxide has found notoriety in the area of superconductivity when it is blended with yttrium, barium and copper oxides. 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, tare the scale with the test tube / beaker on it. Remove the test tube from the beaker and add g g of Copper oxide. 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 g g, 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 OR TA ASSIST YOU WITH THE LIGHTING OF THE GAS THAT EXITS THE TEST TUBE. Experiment

6 Position the ring stand so that the test tube is well away from your face and arms. Hold the Bunsen burner in your right hand and reach behind the ring stand with your left hand to turn on the Gas. Move your lit Bunsen burner under the mouth of the test tube and SLOWLY turn on the gas to the test tube. Adjust the gas flow to the test tube such that the flame is only inches high. 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 minutes (depending on the type of copper oxide used). You will see a copper colored compound form during this heating process. 9. When the entire solid is a copper color, sweep any condensed water out of the test tube by moving the Bunsen burner along the top and bottom of the test tube. 10. 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. 11. After 5 minutes, turn off the gas to the test tube and remove the glass tubing (Caution: HOT!). Allow the test tube to cool to room temperature. 12. Record the mass of the test tube and Copper metal [Data Sheet Q4]. 13. Remove the Copper from the test tube and rub it gently on a hard surface (i.e. a penny) 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. 14. Determine the mass of the Oxygen contained in the Copper oxide [Online Report Sheet Q5]. 15. Determine the mass percent of Copper [Online Report Sheet Q6] and Oxygen [Online Report Sheet Q7] in the Copper oxide. 16. Determine the moles of Copper [Online Report Sheet Q8] and Oxygen [Online Report Sheet Q9] in the Copper oxide. 17. Determine the molar ratio (don t round the answer) between Copper [Online Report Sheet Q10] and Oxygen [Online Report Sheet Q11] in the Copper oxide. 18. Give an empirical formula for Copper oxide (Round To Whole Numbers One Number Will Be A 1 ) [Online Report Sheet Q12]. 19. Write a balanced chemical equation for this reaction assuming you started with Copper (I) oxide [Online Report Sheet Q13]. 20. Write a balanced chemical equation for this reaction assuming you started with Copper (II) oxide [Online Report Sheet Q14]. Experiment

7 Waste Disposal: 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 lab attendees will be permitted to access the online data / calculation submission part of the lab report (click on Lab 10 Empirical Formula 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. Experiment

8 Laboratory 10 Student Data Sheet 1. Mass of the test tube g 2. Mass of the copper oxide g 3. Unknown Number of the Copper Oxide (4 digits) 4. Mass of the test tube and copper metal g Name: Laboratory 10 Instructor Data Sheet Section: 1. Mass of the test tube g 2. Mass of the copper oxide g 3. Unknown Number of the Copper Oxide (4 digits) 4. Mass of the test tube and copper metal g Experiment