Chemistry (12th Edition)

Published by Prentice Hall
ISBN 10: 0132525763
ISBN 13: 978-0-13252-576-3

Chapter 16 - Solutions - 16 Assessment - Page 551: 101


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Work Step by Step

In this question, we design an experiment to calculate the molar mass of an unknown solute based on its freezing point depression. We will need to discuss what measurements need to be made and what calculations will need to be made. A colligative property is one that depends on how many solute particles are dissolved in a solution, not the exact nature of the solute itself. Discussions of strong electrolytes, weak electrolytes, and nonelectrolytes are not entirely relevant except where it can be used to determine the number of particles dissolved. Our discussion will focus on three of these properties. The three colligative properties are as follows: • Vapor pressure lowering - adding solute to a solvent lowers its vapor pressure • Freezing point depression - adding solute to a solvent lowers its freezing point • Boiling point elevation - adding solute to a solvent raises its boiling point The process of solvation creates associations between the solvent molecules and the solute molecules. These small 'shells" of solvation, disrupt the normal process of freezing, requiring greater energy to enforce the normal molecular structure of the solid form of the solvent. These shells also lower the number of particles with sufficient kinetic energy to escape the liquid layer, lowering the vapor pressure of the solution and thus effecting the boiling point in a similar fashion. To accomplish our task, we will measure out a mass of our unknown solute and dissolve it into a known mass of water. So. we will need a balance to measure the solute and a graduated cylinder or balance to measure the water. We will place a thermometer into the solution and then lower the temperature until the solution begins to freeze - this will give us the freezing point of the solution, which should be lower than 0°C. as the freezing point of a solution is lower than the freezing point of the pure solvent as describe above. So. we will need an accurate measurement of the temperature. From this temperature, and the known freezing point depression constant of water (0.512°C/m), we can use the freezing point depression equation to calculate the molality of the solution present. From this molality and the known mass of the water, we can calculate the number of moles of solute present. Finally, we can divide the known mass of the solute by the calculated moles of the solute to determine the molar mass of the solute.
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