Boiling point elevation and molar mass

Molar Mass Determination by Boiling Point Elevation If you dissolve a substance such as ordinary table salt (NaCl) in water, the boiling point of the water will increase relative to the boiling point of the pure water. You can use this property to calculate the molar mass of an unknown. In this assignment, you will dissolve a sample of NaCl in water, measure the boiling point elevation for the solution, and then calculate the molar mass for NaCl as if it were an unknown. 1. Start Virtual ChemLab, select Colligative Properties, and then select Molar Mass Determination by Boiling Point Elevation from the list of assignments. The lab will open in the Calorimetry laboratory with a calorimeter on the lab bench and a sample of sodium chloride (NaCl) on the balance. 2. Record the mass of the sodium chloride in the data table. If it is too small to read, click on the Balance area to zoom in, record the reading, and then return to the laboratory. 3. 100 mL of water is already in the calorimeter. Use the density of water at 25C (0.998 g/mL) to determine the mass from the volume and record it in the data table. Make certain the stirrer is On (you should be able to see the shaft rotating). Click on the green heater light on the control panel to turn on the heater and begin heating the water. Click the clock on the wall labeled Accelerate to accelerate the laboratory time if necessary. 4. Observe the temperature until the first appearance of steam comes from the calorimeter. Immediately click the red light on the heater to turn it off and then record the temperature as the boiling point of pure water in the data table. Letting the water boil will decrease the mass of the water present in the calorimeter. Note that the boiling point may be different than 100C if the atmospheric pressure is not 760 Torr. The current atmospheric pressure for the day can be checked by selecting Pressure on the LED meter on the wall. 5. Drag the weigh paper to the calorimeter and add the NaCl. Wait 30 seconds for the salt to dissolve and then turn on the heater. When steam first appears, observe and record the temperature in the data table. 6. If you want to repeat the experiment, click on the red disposal bucket to clear the lab, click on the Stockroom, click on the clipboard, and select Preset Experiment #2, Boiling Point Elevation – NaCl. Data Table mass NaCl mass water boiling temp of pure water boiling temp of solution 7. Calculate the boiling point elevation, T, caused by adding NaCl to the water. 8. The boiling point elevation can be calculated using the equation T = Kb  m i , where T is the change in boiling point, i is the number of ions in the solution per mole of dissolved NaCl (i = 2), m is the molality of the solution, and Kb is the molal boiling point constant for water which is 0.51C/m. Using this equation and the data recorded in the data table, calculate the molar mass for NaCl and  your answer to 2 decimal places) compare it with the actual value. Show all working (give 9. Restart the experiment. Press save on the thermometer window. Wait one minute and then repeat step 3. Once steam starts to appear leave the water boiling for 2 more minutes. The press stop. Click on the lab book and you will see that your data has saved. Copy and paste the data to an excel spreadsheet. Plot a graph of this data temperature (y axis) vs time. Use the ‘scatter with smooth lines type’ of graph option 10. Restart the experiment. Make a note of the mass of the salt. . Press save on the thermometer window. Wait one minute and then add the salt to the calorimeter. Wait one minute and then turn on the heater. Once steam starts to appear leave the heater on for another two minutes. Press stop. Click on the lab book and you will see your data has saved. Mass of salt used __________________ 11. Export this second set of data to the same excel spreadsheet and plot a new graph, temperature(yaxis) vs time, as a separate chart. What are the major differences between them (you may find it helpful to zoom in on the region of data close to 25 oC and close to 100 oC) ? To do this hover over the y-axis of the chart and then left click. You should see a ‘format axis’ option. Change the min/max initially to 24/26 on each graph. Then change the min/max to 98/102. Make any observations of differences/similarities below. Include in your observations the final temperature of the water and the final temperature of the water when it had salt in it. Differences/similarities between the plots Give an explanation as to the why the pure water and salt solution have different temperature profiles Use the temperature of the salt solution in (11) and your calculated molecular weight to estimate the amount of water left at the end of the experiment. Show all working Graphs to include You do not need to send copies of your graphs to your TA in this lab ...