1. Gas Laws
BHS-PS1-9 Level 3

2. P1V1 T1 P2V2 T2 = Combined Gas Law
* The number of gas particles (moles) is constant in Boyle’s, Charles’s & Gay-Lussac’s Law.
P1V1 T1
P2V2 T2 =
* The remaining three variable (T, P, & V) can be combined into a single equation based on their relationships.

3. P1V1 T1 P2V2 T2 = Key Points * When temperature is constant, pressure
and volume are inversely related (Boyle)
P1V1 T1
P2V2 T2
* When volume is constant, pressure and
temperature are directly related (Gay-Lussac) =
* When pressure is constant, volume and
temperature are directly related (Charles)

4. Temperature must be in Kelvin
Calculations with the Combined Gas Law…….
P1V1 T1
P2V2 T2 =
P1V1T2 = P2V2T1
Temperature must be in Kelvin
Cross out any constants

5. Sample problem
At conditions of 785 torr of pressure and 15.0 °C temperature, a gas occupies a volume of 45.5 mL. What will be the volume of the same gas at 745 torr and 30.0°C?
P1V1T2 = P2V2T1
P1 = 785 torr
V1 = 45.5 mL
T1 = 15.0 °C = 288 K
P2 = 745 torr
V2 = ?
T2 = 30.0 °C = 303 K
P1V1T2 P2T1
V2=

6. P1V1T2 P2T1
V2=
(785 torr)(45.5 mL)(303 K) (745 torr)(288 K)
V2=
V2= 50.4 mL

7. P1V1T2 = P2V2T1 V2T1 V1 T2= Sample problem
On a cold morning (10.0 °C) a group of hot-air balloonists start filling their balloon with air, using a large fan. After the balloon is three-fourths filled, they turn on the propane burner to heat the air. At what Celsius temperature will the air completely fill the envelope to its maximum capacity of m3?
Pressure is constant
P1V1T2 = P2V2T1
V1 = ¾ x m3 =
T1 = 10.0 °C = 283 K
V2T1 V1
V2 = m3
T2 = ?
T2=

8. V2T1 V1
T2=
(1700. m3)(283 K) 1275 m3
T2=
T2=377 K
T2=377 K −273=104°C

9. Ideal versus Real Gases
An ideal gas adheres to the Kinetic Molecular Theory exactly in all situations.
Real gases deviate from ideal behavior at high pressures and low temperatures.
When the pressure is high, it becomes more difficult to compress a gas because the particles actually have a volume of their own.
When the temperature is low, gas particles slow down and attractions between them become significant as they clump together and form liquids.

10. P V = n R T Ideal Gas Law * R is the universal gas constant.
* An “R” value is picked based upon the unit being used
to measure pressure.

11. NOTICE! * If atm is the unit for pressure R = 0.0821
atm L mol K
* If kPa is the unit for pressure R = 8.314
kPa L mol K
* If mmHg is the unit for pressure R = 62.4
mmHg L mol K
In every variation of the R constant, L is the unit of measurement for volume.
Any volume measurement used in this equation must be in liters!
NOTICE!

12. Sample problem
How many moles of a gas at 100.°C does it take to fill a 1.00L flask to a pressure of 1.50 atm?
P V = n R T
P = 1.50 atm
V = 1.00 L
n = ?
R =
T = 100.°C = 373 K
atm L mol K
PV RT n=

13. Option 1 (1.50atm)(1.00L) (0.0821 )(373 K) n= n= 0.0490 mol Option 2
atm L mol K
n= mol
Option 2
separate R when doing your calculations… If R is on the bottom flip the units
(1.50atm)(1.00L) 373 K n= X
mol K atm L
n= mol

14. P V = n R T nRT P V= Sample problem
What is the volume occupied by 9.45 g of C2H2 at STP?
P V = n R T
P = 1.00 atm
V = ?
n = ?
R =
T = 0°C = 273 K
atm L mol K
nRT P V=
***use molar mass to convert g to mol***
9.45g C2H2 1
1 mol g X
= mol C2H2

15. Option 1 (.362958mol)(0.0821 )(273K) 1.00atm V= 8.14 L V= Option 2
atm L mol K
V= 8.14 L
Option 2
separate R when doing your calculations… If R is on top, don’t flip the units
( mol)(273K) 1.00atm V= X
atm L mol K
V= 8.14 L