Clark College Meteorology
Gas Laws
2/6/16
Gas Laws for the Atmosphere
Purpose: To understand the relationship between density, volume, pressure, and
temperature for gases like air.
In this activity a pressure gauge is connected to a container (syringe). The
container is closed so that the amount of air inside the container is fixed.
The approximate mass of air inside a 12.0 cm3 container is: m= 0.012g = 12 mg
I.
Prediction: If the container's volume decreases, will the pressure Increase or
Decrease. Try to guess before you actually do the experiment.
Test you prediction:
As the container's volume decreases, the pressure Increases or Decreases.
Was you prediction correct?
II.
The initial pressure is 1.0 atm, the mass of air inside the syringe is 12.0 mg, and
the original volume is 12.0 cm3.
What is the air density inside the container?
(density = mass /Volume)
=_____________
(remember your units)
Prediction:
Predict the pressure for each Volume (or air density) and record your prediction in the
table below.
Fill in the table below for the calculated air density and predicted pressure.
Volume
(cm3)
Density
(mg/cm3)
12.0
6.0
4.0
18.0
24.0
1.0
Pressure
(atm)
Predict
1.0
1
Pressure
(atm) Meas
Clark College Meteorology
Gas Laws
2/6/16
Test your prediction: Now measure the actual pressure for each volume and record
these values in the previous table.
Plot your values of predicted and measured pressure on the graph below for each volume.
Use a different color or different line style for predicted and measure values, and label
which is which.
Questions:
When the volume of a gas is doubled, what happens to the pressure? (Assume that
everything else stays the same.)
When the volume of a gas is halved, what happens to the pressure? (Assume that
everything else stays the same.)
When the volume of a gas is tripled, what happens to the pressure? (Assume that
everything else stays the same.)
When the volume of a gas is but to 1/3 is original value, what happens to the pressure?
(Assume that everything else stays the same.)
2
Clark College Meteorology
Gas Laws
2/6/16
Using the same data, plot your values of predicted and measured pressure on the graph
below for each density.
Questions:
When the density of a gas is doubled, what happens to the pressure? (Assume that
everything else stays the same.)
When the density of a gas is halved, what happens to the pressure? (Assume that
everything else stays the same.)
When the density of a gas is tripled, what happens to the pressure? (Assume that
everything else stays the same.)
When the density of a gas is but to 1/3 is original value, what happens to the pressure?
(Assume that everything else stays the same.)
If you extend your line to include zero pressure what density would this correspond to?
3
Clark College Meteorology
Gas Laws
2/6/16
Temperature and Pressure.
Another closed container is available in the lab. The mass of air inside this
container and its volume do not change, so both the volume and density will not change
in the activity. A hot water bath, room temperature bath, and ice water bath are also
available so you can cool the container down and warm it up. Record the temperature of
each (room, cold and hot water baths) in both °C and K. Remember: TK=TC+273
The pressure inside the container at room temperature is about 15.0 psi [pounds per
square inch].
Prediction: Will the pressure be higher or lower when the container is placed in the cold
water bath?
Prediction: Will the pressure be higher or lower when the container is placed in the hot
water bath?
T(K)
T(°C)
Pressure
(psi) Meas
Cold
room
hot
Observations:
In the table above, record the pressure inside the container when the container is at: room
temperature (should be about 15 °C) the cold water bath, and the hot water bath.
Is the pressure higher or lower when the container is placed in the cold water bath?
Is the pressure higher or lower when the container is placed in the hot water bath?
4
Clark College Meteorology
Gas Laws
2/6/16
Is the ratio of pressures Phot/Pcold, Phot/Proom, and Proom,Pcold somehow related to
temperature ratios?
Calculate these three ratios (in decimal form) and the ratios of temperatures before
answering this question.
T1(K)
T2(K)
T1(°C)
T2(°C)
P1(psi)
P2(psi)
comments
Hot/cold
Hot/room
Room/cold
How about the ratios of Thot/Phot, Troom/Proom, and Tcold/Pcold using °C and K?
T(K)/P T(°C)/P
cold
room
hot
The value of knowing ratios is that if the ratio R=T/P is a constant then the pressure at
any temperature is T is just T/R. For example if R=T(K)/P=20, then the pressure at 480
K is 480K/20=24 psi.
Is the ratio calculated in the table above constant or nearly constant?
Plot your three data points on the graph above. Do they seem to follow a straight line?
5
Clark College Meteorology
Gas Laws
2/6/16
If you extend this line to zero pressure, what temperature would this correspond to?
Estimate the pressure you would expect to have if the temperature dropped to -60 °C.
Show your work. Hint: Look at the ratios of Temperature to pressure (T/P) and see if you
can see a pattern. Record these ratios in the right hand column of the previous table.
P(-60 °C) = _____________________
Estimate the pressure you would expect to have if the temperature increased to 300 °C.
Show your work.
P(300 °C) = _____________________
Our text gives the equation:
P= c T
as the relationship between Pressure (P), density (), and Temperature (T); c is a constant
to make the units match. This equation is called the gas law or sometimes the equation of
state for the atmosphere. It is an essential equation for numerical weather prediction
models.
What happens to the pressure when the temperature doubles? (everything else staying the
same.)
What happens to the pressure when the temperature is halved? (everything else staying
the same.)
For the equation above what units of temperature are assumed?
6
Clark College Meteorology
Gas Laws
2/6/16
Volume and temperature
Prediction:
What would happen to the volume of a gas at constant pressure if it were moved from
cold water to hot water?
Test your prediction:
What happens to the volume of a gas at constant pressure when it is moved from cold
water to hot water? Try this with a closed syringe.
Questions:
A 4 liter flexible container contains air at a pressure of 20 psi. If the container is
squeezed to a size of 2 liters, what is the new pressure?
a) 40 psi b) 10 psi c) 80 psi d) 5 psi
A 4 literflexible container contains air at a pressure of 20 psi. If the container is
expanded to a size of 8 liters, what is the new pressure?
a) 40 psi b) 10 psi c) 80 psi d) 5 psi
A 4 liter flexible container contains air at a pressure of 20 psi and a temperature of 0°C.
If the container is heated to 273 °C and kept at a volume of 4 liters, what is the new
pressure?
a) 40 psi b) 10 psi c) 80 psi d) 5 psi
7