Department of Chemistry and Geosciences

Honors 1084 Lab Spring 2006

Lab Test 2: Key

Name:_________________________________________________ Section:

Answer the following questions to the best of your ability showing all work necessary to obtain the answer. The answer must be given with the correct units and the correct number of significant figures where applicable. Remember to show all your work necessary to obtain the answer. Simply writing the correct answer without showing any work may not receive full credit.

1. (40 points) The freezing point of a solution prepared by dissolving 1.28 g of a solute in 11.62 g of benzene was measured at 4.42°C. If the freezing point of pure benzene is 5.50°C and its K_f value is 5.12°C/m, what is the molar mass of the solute?

The freezing point depression is equal to: DT_f = 5.50°C – 4.42°C = 1.08°C

The molality of solute is equal to:

Multiply the molality by the number of kilograms of benzene to calculate the number of moles of solute:

Divide the number of grams of solute by the number of moles of solute to calculate the molar mass:

2. (30 points) Suppose that a student recorded the freezing point of solution 1°C higher than the actual freezing point of solution. The freezing point of the solvent was recorded correctly. What would be the effect on the student’s calculated value of the molar mass of the solute? Would it be too large or too small? Please explain your choice by showing how this error affects each step in the calculation of the molar mass. Just giving the correct answer without an explanation will get very little credit.

If the freezing point of the solution was read 1°C higher than its actual value, then DT_f would be larger than its actual value. The calculated molality of the solute would then be larger than its actual value since it is equal to DT_f/K_f. The number of moles of solute would be larger than its actual value since it is equal to the molality times the number of kilograms of solvent. The calculated molar mass would end up too small since it is equal to the number of grams of solute divided by the number of moles of solute.
3. (20 points) In today’s experiment, you are determining the rate law for the reaction of iodine with acetone.