FE Physics
Formulas & Explanation

Gases

Kelvin  temperature

T = t + 273

T  Kelvin temperature  in K

t   temperature in oC

Ideal Gas Law

pV = n R T

p  pressure in  Pa

V  volume in m3

n  number of moles

R  gas constant   R = 8.3145 Jmol-1K-1

T  Kelvin temperature  in K

Example

In a tank of 200 is helium. The pressure in the tank is 30 bar and the temperature  is 20 oC.

Calculate the mass of helium.

p V = n R T

(30 x 105 )(200 x 10-3 ) = n (8.31)(293

n = (30 x105   * 200 x 10-3) / (8.31 x 293) = 246 mol

mass per mol of helium is 4.0 g mol-1

mass of helium : 246 x 4 = 985.7 g = 9.9 x 102 g

For a fixed mass of gas is:

( p1V1) /T1 = (p2V2)/T2            (fixed number of moles)

Example

The pressure in a tank of 20 is 3.0 bar at a temperature of 17 oC.

The temperature is raised. The volume increases to 21 and the pressure to 3.5 bar.

Calculate the new temperature.

( p1V1) /T1 = (p2V2)/T2                                     T in Kelvin !

(3.0 x 20) /290 = (3.5 x 21) /T2

60/290 = 73.5/T2

60 T2 = 73.5 x 290

60 T2 = 21315

T2 = 355 K = 82 oC

Special cases of the Gas Law:

Boyle’s law           pV = constant

p1V1 = p2V2         (number of moles and temperature constant)

Gay-Lussac’s law

p/T = constant   ( number of moles and the volume constant)

p1/T1 = p2/T2             Remark  : T in K

Charles’s law         V/T = constant    ( number of moles and pressure constant)

V1/T1=V2/T2              Remark : T in K