Using Gas Laws
• Convert temperatures to Kelvin!
• Ensure volumes and/or pressures are in the same units on both sides of equation.
• STP = 0° C and 1 atm.
• Pressure conversion factors: 1 atm = 760 torr = 760 mm Hg = 101,325 Pa
I. When comparing 2 conditions, use the combined gas law. If one of the variables is held
constant, just cancel it on both sides before you start.
V1P1 = V2P2
T1
T2
II.
K = °C + 273 (no degree symbol)
When dealing with an amount of substance (moles or grams), use the Ideal Gas Law.
PV = nRT
o
o
o
o
Convert temperatures to Kelvin!
Convert volume to L
Choose the R to match the given units of pressure
n is moles, so convert grams to moles using molar mass if grams are given.
R = 0.0821 L- atm
mol-K
and R = 62.4 L-torr
mol-K
Dalton’s Law of Partial Pressures: In a container, the total pressure is the sum of the partial pressures.
If given a mixture of gases, add the pressure provided by each to get total pressure. Find the partial
pressure of a gas using PV = nRT if needed, then add partial pressures of all gases involved to get the total
pressure. (P = nRT/V)
III.
26. A 1.00 liter mixture of gas is produced by mixing nitrogen and argon. If the pressure of the nitrogen alone is
known to be 445 torr and the pressure of the argon alone is known to be 79 kPa, what is the total pressure of the gas
mixture in the flask? (1.04 X 103 torr or 138 kPa)
27. A mixture of helium and oxygen in a 5.0 liter flask is known to have a total pressure of 1.44 atm. If the pressure
of the helium is known to be 668 mm Hg, what is the pressure of the oxygen in torr? (426)
28. 1.00 liter of nitrogen at 200 torr is mixed with 2.5 liters of oxygen at 300 torr in a new container whose volume
is 1.5 liters. What is the total pressure of the mixture of nitrogen and oxygen in the 1.5 liter container? (633 torr)
29. 250 ml of neon whose temperature is 25 oC and whose pressure is 750 torr is mixed with 500 ml of nitrogen
whose temperature is 22oC and whose pressure is 555 torr and the mixture is placed in a new container whose
volume is 10.0 liters. What is the pressure of the mixture in the new container if the temperature of the new
container is raised to 100oC?
26. A 1.00 liter mixture of gas is produced by mixing nitrogen and argon. If the pressure of the nitrogen alone is
known to be 445 torr and the pressure of the argon alone is known to be 79 kPa, what is the total pressure of the gas
mixture in the flask? (1.04 X 103 torr or 138 kPa)
79 kPa | 760 torr
| 101.325 kPa
= 592 torr + 445 torr = 1038 torr
27. A mixture of helium and oxygen in a 5.0 liter flask is known to have a total pressure of 1.44 atm. If the pressure
of the helium is known to be 668 mm Hg, what is the pressure of the oxygen in torr? (426)
668 mm Hg | 1 atm
atm
| 760 mm Hg
= 0.879 atm from He. Pt = PHe+ PO2 so PO2 = PT – PHe = 1.44 atm – 0.879 atm = .561
.561 atm | 760 torr= 426 torr
| 1 atm
28. At 273 K, 1.00 liter of nitrogen at 200 torr is mixed with 2.5 liters of oxygen at 300 torr in a new container
whose volume is 1.5 liters. What is the total pressure of the mixture of nitrogen and oxygen in the 1.5 liter
container? (633 torr)
One way to do this:
1. Find total moles gas
2. Use PV = nRT with new volume and total moles gas
n = PV = 200 torr(1.00 L)
= 0.0117 mol N2 n = PV = 300 torr(2.5 L)
= 0.044 mol O2
RT (62.4 L-torr mol-1K-1)(273K)
RT (62.4 L-torr mol-1K-1)(273K)
Molestotal= .0117 + .044 = 0.0557
Pt = .0557 moles(62.4 L-torr)(273K) = 633 torr
1.5 L (mol-K)
29. 250 ml of neon whose temperature is 25 oC and whose pressure is 750 torr is mixed with 500 ml of nitrogen
whose temperature is 22oC and whose pressure is 555 torr and the mixture is placed in a new container whose
volume is 10.0 liters. What is the pressure of the mixture in the new container if the temperature of the new
container is raised to 100oC? (58 torr)
One way to solve: 1. Use PV=nRT to find moles of each gas.
2. Use PV = nRT with total moles, new volume and new temp.
n = PV = 750 torr(.25 L)
= 0.010 mol Ne
RT (62.4 L-torr mol-1K-1)(298 K)
Molestotal= .010 + .015 = 0.025 mol
n = PV = 555 torr(.50 L)
= 0.015 mol N2
RT (62.4 L-torr mol-1K-1)(295K)
Pt = .025 moles(62.4 L-torr)(373K) = 58 torr
10.0 L (mol-K)