CHEM 1451 Lab Week 09 Colligative Properties Postlab Report. Coach s Name

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1 Instructions To complete the table entry titled Measured, copy your measurement that you made in the previous week. If you ran the experiment again to obtain a different measurement, then check the box above the table so that your Coach will know why the numbers are different from the ones reported in the Prelab and Data. Although there will be no points for completing a table entry, one point will be deducted if the data is missing or for the incorrect use of significant figures and units. So make sure to write them down correctly. Part A: Preparing Solutions Solution B Measured Mass of NaCl Measured Volume of Water 1. Answer each question with correct significant figures and units. You must show calculations for determining each quantity. If necessary, use the molar mass of NaCl = g/mol. a. Determine the number of moles of NaCl in the solution. (5 points) b. Determine the number of moles of all solutes (both cations and anions) from the number of moles of NaCl that you determined above. For this question, in addition to your calculation, explain the idea behind your calculation. (5 points) c. Determine the mass (in kg) of solvent (assume that the density of water is g/ml). (5 points) d. Determine the molality (in m or mol/kg) of all solutes. (5 points) 1

2 Solution C Measured Mass of NaCl Measured Volume of Water 2. Answer each question with correct significant figures and units. You must show calculations for determining each quantity. If necessary, use the molar mass of NaCl = g/mol. a. Determine the number of moles of NaCl in the solution. (5 points) b. Determine the number of moles of all solutes (both cations and anions) from the number of moles of NaCl that you determined above. For this question, in addition to your calculation, explain the idea behind your calculation. (5 points) c. Determine the mass (in kg) of solvent (assume that the density of water is g/ml). (5 points) d. Determine the molality (in m or mol/kg) of all solutes. (5 points) 2

3 Solution D Measured Mass of NaCl Measured Volume of Water 3. Answer each question with correct significant figures and units. You must show calculations for determining each quantity. If necessary, use the molar mass of NaCl = g/mol. a. Determine the number of moles of NaCl in the solution. (5 points) b. Determine the number of moles of all solutes (both cations and anions) from the number of moles of NaCl that you determined above. For this question, in addition to your calculation, explain the idea behind your calculation. (5 points) c. Determine the mass (in kg) of solvent (assume that the density of water is g/ml). (5 points) d. Determine the molality (in m or mol/kg) of all solutes. (5 points) 3

4 Part C: Measuring the Freezing Point of Water Measure Freezing Point of Water Part D: Measuring the Freezing Point of NaCl Solutions Measured Freezing Point of Solution B Measured Freezing Point of Solution C Measured Freezing Point of Solution D 4. Using the measured freezing point of distilled water in Part C, calculate the freezing point depression T for NaCl solutions from Part D. The freezing point depression T is calculated by the following formula. T = (Measured freezing point of water in Part C) (Measured freezing point in Part D) a. Determine the freezing point depression T for Solution B. Show your calculation. (5 points) b. Determine the freezing point depression T for Solution C. Show your calculation. (5 points) c. Determine the freezing point depression T for Solution D. Show your calculation. (5 points) 4

5 5. Determine the molal freezing point depression constant (K f) (in C kg/mol) of water by using the data of three NaCl solutions. Use Equation 3 in the Background and Procedure file, the molality values of all solutes from Questions 1-3 and the freezing point depression ( T) from Question 4. In your calculation, rewrite the symbol m (the molality) as mol/kg so that the molal freezing point depression constant will have the correct unit. Make sure to show all your work to receive credits. a. Use the data of Solution B and determine the molal freezing point depression constant (K f) (in C kg/mol) of water. (5 points) b. Use the data of Solution C and determine the molal freezing point depression constant (K f) (in C kg/mol) of water. (5 points) c. Use the data of Solution D and determine the molal freezing point depression constant (K f) (in C kg/mol) of water. (5 points) 5

6 6. Analysis of Results a. Determine the average value of three molal freezing point depression constant (K f) values of water from Question 5 (in C kg/mol). (5 points) b. The published value for K f of water is 1.86 C kg/mol (use this as the true value). Determine the % error of the average value of the molal freezing point depression constant that you found in Question 6 a. average value true value %error = 100% true value The % error is an example of the mixed operation. In order to receive the full credits, make sure to show each step of calculations. In other words, show the result of the calculation in the numerator explicitly before dividing the numbers. (5 points) 6