1.
Weigh your sample of Pb shot. (**You
should use no less than 50 grams.**) Measure
room temperature.

2.
Place the outer can of the calorimeter with its insulating (large)
Styrofoam cup and insulating ring on the balance.
Put the Pb sample in a small Styrofoam cup and place it on the pan next
to the calorimeter. The total mass of the system will eventually include the
mass of a Styrofoam cup with liquid nitrogen, but you may want to balance the
scales now.

3.
With the timer, and with pencil and paper at hand, be ready to record the
mass on the balance as a function of time.
You will later want to plot mass on the balance versus time.
Your mass will *decrease* by around 50 grams during the experiment.
**Thus your mass axis should be expanded to include only this 50 grams,
from the total starting mass of your whole system to the total finishing mass.**

4.
Obtain a sample of liquid nitrogen in your small Styrofoam cup and place
the Styrofoam cup into the ring on the calorimeter.
Rebalance the scale. Your
weights should contain several 10 g and 20 g masses that can be removed as the
nitrogen evaporates. After you have
achieved a balance, move the sliding mass down to the next lowest even integer
mass (i.e., 0,2,4,6,8). This will
unbalance the scale. As the
nitrogen evaporates, the pointer will swing back through the balance point.
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 to move the sliding mass down in 2-g steps and record the times
when the pointer goes through the balance point.
When the sliding mass gets to 0 you will need to remove 10 g from the pan
and move the slidewire mass to the 8-g mark.

6.
When you have 6 or 7 points, it should be obvious from your data that
your plot will be a straight line. 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 glass
beaker back on the balance pan. The
nitrogen will boil rapidly and you should try to take data again as soon as you
can, i.e., mass versus time. You
will need to remove mass (more than two grams) from the pan to regain your
equilibrium. *Be
sure to record the amount of mass removed at this time**!*

7.
Continue taking data until you again have 6 to 8 points. The slope of this data is your new background loss rate.

8.
Call up Excel and enter your data. The plot 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

9. 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.

10.
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

11.
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?**