melting points of a mixture

melting points of a mixture - CER Modular Laboratory...

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Measuring the Melting Points of Compounds and Mixtures prepared by Joseph W. LeFevre, SUNY, Oswego Measure the melting points of pure benzoic acid and pure mandelic acid. Determine the eutectic composition and the eutectic temperature of benzoic acid–mandelic acid mixtures. Identify an unknown compound using mixture melting points. None The melting point of a compound is the temperature at which the solid is in equilibrium with its liquid. A solid compound changes to a liquid when the molecules acquire enough energy to overcome the forces hold- ing them together in an orderly crystalline lattice. For most organic com- pounds, these intermolecular forces are relatively weak. The melting point range is defined as the span of temperature from the point at which the crystals first begin to liquefy to the point at which the entire sample is liquid. Most pure organic compounds melt over a narrow temperature range of 1–2 °C. The presence of a soluble impurity almost always causes a decrease in the melting point expected for the pure compound and a broadening of the melting point range. In order to understand the effects of impuri- ties on melting point behavior, consider the melting point–mass percent composition diagram for two different fictitious organic compounds, X and Y, shown in Figure 1. The vertical axis represents temperature and the horizontal axis represents varying mass percent compositions of X and Y. Both compounds have sharp melting points. Compound X melts at 150 ° C, as shown on the left vertical axis, and Y melts at 148 ° C, as shown on the right vertical axis. As compound X is added to pure Y, the melt- ing point of the mixture decreases along curve CB until a minimum tem- perature of 130 ° C is reached. Point B corresponds to 40 mass percent X and 60 mass percent Y and is called the eutectic composition for com- pounds X and Y. Here, both solid X and solid Y are in equilibrium with the liquid. The eutectic temperature of 130 ° C is the lowest possible melting point for a mixture of X and Y. At temperatures below 130 ° C, mixtures of X and Y exist together only in solid form. C E R · M o d u l a r · L a b o r a t o r y · P r o g r a m · i n · C h e m i s t r y editor: Joe Jeffers Copyright © 1997 by Chemical Education Resources, Inc., P.O. Box 357, 220 S. Railroad, Palmyra, Pennsylvania 17078 No part of this laboratory program may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy- ing, recording, or any information storage and retrieval system, without permission in writing from the publisher. Printed in the United States of America. 00 99 98 97 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 TECH 701 PURPOSE OF THE EXPERIMENT BACKGROUND REQUIRED BACKGROUND INFORMATION Figure 1 Melting point–mass percent composition diagram for a two- component mixture 150 146 130 100 0 80 20 60 40 40 60 20 80 0 10 temperature , °C A C B solid + solid X Y liquid + solid X liquid + solid Y liquid + liquid X Y mass % mass % X Y
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Consider a 100-microgram ( µ g) mixture composed of 20 µ g of X and 80 µ g of Y.
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