Sunday 17 March 2019

Air - Composition and Molecular Weight

We did wonderful discussion with SHRI JK PRAAD SIR

Components in Dry Air

Air is a mixture of several gases, where the two most dominant components in dry air are 21 vol% oxygen and 78 vol% nitrogen. Oxygen has a molar mass of 15.9994 g/mol and nitrogen has a molar mass of 14.0067 g/mol. Since both of these elements are diatomic in air - O2 and N2, the molar mass of oxygen gas is 32 g/mol and the molar mass of nitrogen gas is 28 g/mol.
The average molar mass is equal to the sum of the mole fractions of each gas multiplied by the molar mass of that particular gas:
Mmixture = (x1*M1 + ......+ xn*Mn)                                        (1)
where
x= mole fractions of each gas
Mi = the molar mass of each gas
The molar mass of dry air is 28.9647 g/mol.  Composition and content of each gas in air is given in the figures and the table below.
Air - composition

Minor components dry air

Air is usually modeled as a uniform (no variation or fluctuation) gas with properties averaged from the individual components.
Components in dry airVolume ratio = Molar ratio
compared to dry air
Molar massMolar mass in airAtmospheric
boiling point
NameFormula[mol/molair][vol%][g/mol],
[kg/kmol]
[g/molair],
[kg/kmolair]
[wt%][K][°C][°F]
NitrogenN20.7808478.08428.01321.87398375.5277.4-195.8-320.4
OxygenO20.2094620.94631.9996.70246923.1490.2-183.0-297.3
ArgonAr0.009340.93439.9480.3731141.2987.3-185.8-302.5
Carbon dioxideCO20.000330.03344.0100.0146770.051194.7-78.5-109.2
NeonNe0.000018180.00181820.1800.0003670.001327.2-246.0-410.7
HeliumHe0.000005240.0005244.0030.0000210.000074.2-269.0-452.1
MethaneCH40.000001790.00017916.0420.0000290.00010111.7-161.5-258.7
KryptonKr0.00000100.000183.7980.0000840.00029119.8-153.4-244.0
HydrogenH20.00000050.000052.0160.0000010.00000320.3-252.9-423.1
XenonXe0.000000090.000009131.2930.0000120.00004165.1-108.1-162.5
Average molar mass of air28.9647
  • The water or vapor content in air varies. The maximum moisture carrying capacity of air depends primarily on temperature
  • The composition of air is unchanged until elevation of approximately 10.000 m
  • The average air temperature diminishes at the rate of 0.6oC for each 100 m vertical height
  • "One Standard Atmosphere" is defined as the pressure equivalent to that exerted by a 760 mm column of mercury at 0osea level and at standard gravity (32.174 ft/sec2)
Other components in air
  • Sulfur dioxide - SO2 - 1.0 parts/million (ppm)
  • Nitrous oxide - N2O - 0.5 parts/million (ppm)
  • Ozone - O3 - 0 to 0.07 parts/million (ppm)
  • Nitrogen dioxide - NO2 - 0.02 parts/million (ppm)
  • Iodine - I2 - 0.01 parts/million (ppm)
  • Carbon monoxide - CO - 0 to trace (ppm)
  • Ammonia - NH3 - 0 to trace (ppm)

Common Pressure Units frequently used as alternative to "one Atmosphere"

  • 76 Centimeters (760 mm) of Mercury
  • 29.921 Inches of Mercury
  • 10.332 Meters of Water
  • 406.78 Inches of Water
  • 33.899 Feet of Water
  • 14.696 Pound-Force per Square Inch
  • 2116.2 Pounds-Force per Square Foot
  • 1.033 Kilograms-Force per Square Centimeter
  • 101.33 kiloPascal


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