Pearson Education Limited Edinburgh Gate Harlow Essex CM20 2JE England and Associated Companies throughout the world

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2 Pearson Education Limited Edinburgh Gate Harlow Essex CM20 2JE England and Associated Companies throughout the world Visit us on the World Wide Web at: Pearson Education Limited 2014 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without either the prior written permission of the publisher or a licence permitting restricted copying in the United Kingdom issued by the Copyright Licensing Agency Ltd, Saffron House, 6 10 Kirby Street, London EC1N 8TS. All trademarks used herein are the property of their respective owners. The use of any trademark in this text does not vest in the author or publisher any trademark ownership rights in such trademarks, nor does the use of such trademarks imply any affiliation with or endorsement of this book by such owners. ISBN 10: ISBN 10: ISBN 13: ISBN 13: British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Printed in the United States of America

3 tubing with water by loosening the pinch clamp, attaching a rubber bulb to tube B, and applying pressure through it. Close the clamp when the tube is filled. GIVE IT SOME THOUGHT Determination of R: The Gas Law Constant Why can t tube B extend below the water level in the bottle? Mix the solids in the test tube by rotating the tube, making sure none of the mixture is lost from the tube, and attach tube B as shown in Figure 1. (CAUTION: When you attach the test tube, make sure that none of the KClO 3 and MnO 2 comes in contact with the rubber stopper; otherwise, or a severe explosion may result. Make certain that the clamp holding the test tube is secure so that the test tube does not move.) Fill the beaker about half full of water, insert glass tube A, open the pinch clamp, and lift the beaker until the levels of water in the bottle and beaker are identical. Then close the clamp, discard the water in the beaker, and dry the beaker. The purpose of equalizing the levels is to produce atmospheric pressure inside the bottle and test tube. Set the beaker with tube A in it on the desk and open the pinch clamp. A little water will flow into the beaker, but if the system is airtight and has no leaks, the flow will soon stop and tube A will remain filled with water. If this is not the case, check the apparatus for leaks and start over. Keep in the beaker the water that has flowed into it; at the end of the experiment, the water levels will be adjusted and this water will flow back into the bottle. Heat the lower part of the test tube gently (make certain the pinch clamp is open) so that a slow but steady stream of gas is produced, as evidenced by the flow of water into the beaker. When the rate of gas evolution slows considerably, increase the rate of heating, and heat until no more oxygen is evolved. Allow the apparatus to cool to room temperature, making sure the end of the glass tube in the beaker is always below the surface of the water. Equalize the water levels in the beaker and the bottle as before and close the clamp. Weigh a 125 ml Erlenmeyer flask to the nearest 0.01 g and empty the water from the beaker into the flask. Weigh the flask with the water in it. Measure the temperature of the water and using the density of water in Table 1, calculate the volume of the water displaced. This is equal to the volume of oxygen produced. Remove the test tube from the apparatus and accurately weigh the tube and its contents. The difference in mass between this and the original mass of the tube plus MnO 2 and KClO 3 is the mass of the oxygen produced. GIVE IT SOME THOUGHT How is the volume of water displaced equal to the volume of oxygen produced? Record the barometric pressure. The vapor pressure of water at various temperatures is also given in Table 1. Or Styrofoam cup. The volume of water may also be measured directly but less accurately with a graduated cylinder. 213

4 Determination of R: The Gas Law Constant TABLE 1 Density and Vapor Pressure of Pure Water at Various Temperatures Temperature ( C) Density (d) (g/ml) Temperature ( C) HO 2 vapor pressure (mm Hg) Waste Disposal Instructions KClO 3 is a powerful oxidizing agent and must not be disposed of in a waste basket! Do not attempt to clean out the residue that remains in the test tube. Return the test tube to your instructor or follow his or her instructions for disposal of its contents. Calculate the gas law constant, R, from your data, using the ideal gas equation. 2 2 Calculate R using the van der Waals equation ( P + n a/v )( V nb) = nrt (for 2 2 O 2, L atm/mol and units straight cm / mol). Make sure you keep your Error Analysis Determine the maximum and minimum values of R consistent with the experimental reliability of your data from the ideal gas law. PV (3200. g/mol) PV R = = nt mt GIVE IT SOME THOUGHT Before performing any calculations, what do you expect R to be? Assume that the reliabilities for the various measured quantities in this experiment are as follows: P =± 0. 1 mm Hg T =± 1 C V =± L m =± g To determine the maximum value of R, use the maximum values that the pressure and volume may have and the minimum values that the mass and temperature may have. Similarly, calculate the minimum value of R from the minimum values of P and V and the maximum values for m and T. Determine the average value of R and assign an uncertainty range to this average value. 214

5 EXAMPLE 1 Assume that the measured quantities were as follows: P = mm Hg, T = 20 C, V = ml, and m = g. What would be the maximum and minimum values of R, the average value of R, and the uncertainty range assigned to this average value? SOLUTION: First, put the measured quantities into proper units as follows: Therefore, Determination of R: The Gas Law Constant mm Hg P = = atm 760 mm Hg/atm V = ml = L m = g T = (20 C + 273) K = 293 K [ ] mm Hg/(760 mm Hg/atm) ( L)( g/mol) Maximum R = ( g)(292 K) = L{ atm/mol{ K [ mm Hg/(760 mm Hg/atm) ]( L)( g/mol) Minimum R = ( g)(294 K) = L{ atm/mol{ K Therefore, the average value for R is as follows: Average R = L-atm/mol-K 2 = L-atm/mol-K Note that the minimum and maximum values of R differ from the average by Consequently, the uncertainty in R can be written as ± L-atm/mol-K and the data would be reported as R = ± L-atm/mol-K 215

6 NOTES AND CALCULATIONS 216

7 Name Desk Date Laboratory Instructor Determination of R: The Gas Law Constant Pre-lab Questions Before beginning this experiment in the laboratory, you should be able to answer the following questions. 1. Under what conditions of temperature and pressure would you expect gases to obey the ideal gas equation? 2. Calculate the value of R in L-atm/mol-K by assuming that an ideal gas occupies L/mol at STP. 3. Why do you equalize the water levels in the bottle and the beaker? 4. Why does the vapor pressure of water contribute to the total pressure in the bottle? 5. What is the value of an error analysis? 6. Suggest reasons why real gases might deviate from the ideal gas law on the molecular level. 217