Chapter 18
The Combined Gas Law
&
Graham’s Law
Combined Gas Law
• 10.0 cm3 volume of a gas is collected at 75.6 kPa
and 60.0C. What would its volume be at STP?
6.12 cm3
Combined Gas Law Problem
A sample of helium gas has a volume of
0.180 L, a pressure of 0.800 atm and a
temperature of 29°C. What is the new
temperature(°C) of the gas at a volume of
90.0 mL and a pressure of 3.20 atm?
Combined Gas Law Problem
A sample of helium gas has a volume of
0.180 L, a pressure of 0.800 atm and a
temperature of 29°C. What is the new
temperature(°C) of the gas at a volume of
90.0 mL and a pressure of 3.20 atm?
302 K
x
3.20 atm
0.800 atm
x
90.0 mL = 604 K
180.0 mL
604 K - 273 = 331 °C
Weather Balloons
Weather Balloons
• A weather balloon is an instrument which measures
temperature, humidity, and atmospheric pressure. The
weather balloon is generally a balloon mounted with
weather measuring equipment and a radio transmitter.
Some weather balloons are equipped with GPS or global
positioning systems for the weather station to track it more
accurately.
• In North America prior to release the balloon is usually
filled with hydrogen gas due to lower cost, though helium
can be used as a substitute. Weather balloons may reach
altitudes of 40 km (25 miles) or more where the diminishing
pressure eventually causes the balloon to expand to such
a degree that it pops.
• Weather balloons are sometimes cited as the cause for
unidentified flying object sightings.
A weather balloon with a volume of 305L at STP is released
and rises until it expands and explodes at a pressure of
7.00 torr and a temperature of –21°C. What was the volume
of the balloon when it burst?
≈ 30,600 L
A weather balloon with a volume of 305L at
STP is released and rises until it expands and
explodes at a pressure of 7.00 torr and a
temperature of –21°C. How many times larger
is the balloon than when it was released?
About 100 times larger
Diffusion
• The spreading of a gas from an area of
higher concentration to an area of lower
concentration.
Which gas would diffuse
throughout a room faster?
NH3(g) or HCl(g)
Gas Diffusion and Effusion
• Graham's Law:
• Allows for the calculation of the rate of
diffusion of gas molecules.
Gas Diffusion and Effusion
Graham's Law:
governs the rate of diffusion of gas molecules.
Rate of A
molar mass of B

Rate of B
molar mass of A
Calculate the rate of diffusion of NH3(g) compared to HCl(g)
Rate of A
molar mass of B

Rate of B
molar mass of A
NH3 diffuses 1.47 times faster than HCl
Use measurement to confirm the rate of
diffusion of NH3(g) compared to HCl(g)
HCl
NH3
Calculate the rate of diffusion of helium compared to argon.
Rate of A
molar mass of B

Rate of B
molar mass of A
Helium diffuses 3.2 times faster than argon
Calculate the rate of diffusion of hydrogen compared to neon.
Rate of A
molar mass of B

Rate of B
molar mass of A
hydrogen diffuses 3.2 times faster than neon
Homework (All due in 2 days)
• Summarize the (I) Purpose and (II)
Procedure of the “Gas Laws” Lab.
• Study Guide Chapter 18.
• Worksheet: Combined Gas Law &
Graham’s Law.