PASCO TD-8565 Adiabatic Gas Law Apparatus - Manual (Page 22)

© 2009 Physics Enterprises 22
2. Close all the valves and hold the lever at the highest position. Set the computer interface
(Science Workshop, PASCO) to record temperature on the x-axis and pressure on the
y-axis. Initiate computer acquisition and push the lever down quickly for a rapid
compression of the gas in the chamber.
3. Use the curve-fit program in the software to determine the slope of the displayed trace of
pressure vs. temperature. This will be the experimental value for ΔP/ΔT.
Calculations
1. Multiply the ΔP/ΔT by 2.29 † to convert from the voltage slope to kPa/K, and then multiply
by 0.00986 to obtain the unit of atm/K. This calculated ΔP/ΔT will be approximately (∂P/∂T)
S
and can be combined with the ΔV/ΔT (∂V/∂T)
p
from another experiment to determine C
p
.
2. Read the height (cm) of the chamber and calculate its volume by
V (cc) = height · Π · (4.51/2) 2
3. Use the perfect gas equation (PV=nRT), the calculated chamber volume, the ambient
temperature, and the barometric pressure of the day to estimate the number of mole of
the gas in the chamber.
4. Report C
p
in j/K/mol.
Reliability of Results
The calculated results from experiments over many years have been comparable to
literature values. The gases used have been argon and carbon dioxide. For argon, the
average experimentally determined C
p
from the years 1995 to 2006 is 31 j/K/mol, while the
literature value is 20.8 j/K/mol. For carbon dioxide, the average over the same period of time
is 41 j/K/mol, and the literature value is 37 j/K/mol. The typical uncertainty in each reported
result has been between 10 and 20 percent.
† Pressure and temperature calibrations of the Adiabatic Gas Law Apparatus.