1. LECTURE UNIT NO. 6
Ideal Gas Law
- Equation of state for an ideal gas. This equation closely predicts the behavior of real gasses
when gasses are not approaching the point where they condense into a liquid.
In terms of mass
In terms of moles (mol)
R = gas constant, each ideal gas has its own gas constant.
Rair = 0.287 kJ/ kg-K SI Units
Rair = 1716 _ft - lbf_ Eng’g Units
lbm °R
Note:
A gas can be considered ideal if the pressure is very low and the temp is much higher than its
critical temperature
Gas Constants:
Where: MW = molecular weight
Element is defined by:
n Cx
Compound Element is defined by:
n Cx Hy
Common Elements Used
Elements Atomic Weight (AW) Number of Atoms Molecular Weight (MW)
C (Carbon) 12 1 12
H2 (Hydrogen) 1 2 2
O2 (Oxygen) 16 2 32
N2 (Nitrogen) 14 2 28
S (Sulfur) 32 1 32
Common Compound Elements Used
Elements Atomic Weight (AW) Number of Atoms Molecular Weight (MW)
CO2 (Carbon dioxide)
CO (Carbon monoxide)
H2O (Water Vapor)
SO2 (sulfur Dioxide)
SO (Sulfur Monoxide)

2. Universal Gas Constant
According to Avogadro’s Law
n V t P___________
1 pmole 359 ft3 32°F 14.7 psia
1 kgmole 22.4 m3 0 °C 101.325 kPa_____
R = universal gas constant = 8.314 kJ/ kgmole-K = 1545 1716 _ft lb__
pmole °R
MOIST or HUMID AIR
Illustration:
Moist or Humid Air = Dry Air + Water Vapor
O2 N2 H2Oair
In terms of mass,
m M = ma + m V