II. Procedure

1.     Weigh your sample of Pb shot.  (You should use no less than 100 grams.)  Measure room temperature.

2.    Place the outer can of the calorimeter with its insulating (large) Styrofoam cup and lid on the balance.  Put the Pb sample in a small Styrofoam cup and place it on the pan next to the calorimeter.

3.    We are going to be connecting to the Ohaus balance directly.  Open DataStudio and click Create Experiment.  Click on the “Choose Interface” button.  Select the PASPORT bullet in the dialog box and click OK.  Click the setup button, click the “Add Sensor or Instrument” button, and choose “Instruments” from the pull-down menu.  Double click on the Ohaus Balance.  A graph appears.  Move this graph to the bottom of the page.  Make sure there is no yellow exclamation point in the icon for the balance.  Once it goes away you are ready to take data by pressing start.   Also add a data table to the experiment.

4.   Obtain a sample of liquid Nitrogen in your large Styrofoam cup and place the Styrofoam cup into the calorimeter.  At this time, start the clock.  The clock will continue to run for the remainder of the experiment. The total mass on the balance at this time is your t = 0 mass and should be plotted on your graph as such.

5.    Continue running until you have lost about 20 grams of liquid Nitrogen. The slope of this data gives you a background loss rate.  At this point, transfer your Pb sample into the liquid Nitrogen as quickly as possible (the clock is still running) but without splashing any Nitrogen or putting your fingers in it.  Be sure to note the time when you add the Pb and remember to put the empty small cup back on the balance pan.  The Nitrogen will boil.  Note the time at which you believe all of the Pb is at 77K.  

6.    Continue taking data until you have lost another 20 grams of Nitrogen.  The slope of this data is your new background loss rate.  Get a printout of the DataStudio graph.

7.    Move your data into Excel. The plot you want will be one of mass versus elapsed time. However, we want two separate data series on the same graph, one for each background rate, so that we can get two separate best-fit lines. Enter your data from before you added the Pb as one series and enter the data from after you added the Pb as the other series. Now obtain two least squares fits to the two background loss rates; i.e., fit the first set of points separately to a straight line and then fit the second set of points to a straight line.  You can extend these lines by right-clicking on the line and selecting format trendline.  Then under options, select forecast forward/backward about 100 units.  Get a printout of the DataStudio graph and the graphs from the spreadsheet.

8.    Using the graph that you have just printed out, you will now determine the amount of liquid Nitrogen that was lost when you added the Pb. You want the best value for the mass that you can get.  You can obtain this mass numerically by using the transfer time in the equations for your best-fit lines and then taking the difference between the mass before and the mass after.  Alternatively, you can obtain the change in mass graphically by following the procedure in step 10 below.

9 .  Expand the y-axis to be as large as possible. This will mean that some of your data may not show up on this new graph (which is why you printed out the original graph in part 8). Right-click on the y-axis of your graph. Choose format axis. In the Patterns menu click the option for minor tick marks (inside). Then, go to the Scale menu to format the minor tick marks and the scale of your graph. Choose 1 for the size of the minor unit for tick mark. Now format your y-axis so that it just includes the points of interest, namely, the last few points from before the Pb was added and the first few points after the Pb was added. Get a printout of this graph.  On this new graph, choose a time midway between when you transferred the Pb and when the background rate was re-established. Draw a vertical line at this time. The points at which this vertical line intersects the two background lines define the mass of nitrogen that boiled off to cool the metal. This mass is read off the vertical axis.

10.  Using the specific heat of Pb and its change in temperature, you can find the amount of heat lost by the Pb and hence the heat absorbed by the liquid nitrogen.  Using the heat absorbed by the liquid nitrogen and the mass of the nitrogen boiled off by the lead, calculate the latent heat of vaporization of nitrogen and compare it to the book value of 197.7 J/g.  Compare to your measured value by calculating the percent error.  How do you account for any discrepancies?  Do the problems you've considered make the agreement better or worse?