TITLE.PM5

GASES AND VAPOUR MIXTURES 413

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Mean molecular weight of air = 28.96
Gas constant R = 0.287 kJ/kg K
For approximate calculations the air is said to be composed of oxygen and ‘atmospheric nitro-
gen’.

Constituent Molecular weight Volumetric analysis % Gravimetric analysis %

Oxygen 32 21 23.3

Atmospheric nitrogen 28 79 76.7

Nitrogen/Oxygen — 3.76 : 1 3.29 : 1

Note. Volumetric analysis is the analysis by volume ; gravimetric analysis is the analysis by weight
or mass.

9.3. Volumetric Analysis of a Gas Mixture

It is usual practice to quote the analysis of a mixture by volume as this is the most convenient
for practical determinations. In article 11.15, the analysis of exhaust or flue gases by means of the
Orsat apparatus is described. The volume of the gas sample is measured at atmospheric pressure,
and the temperature is held constant by means of a water jacket round the gas sample. The
constituents are absorbed chemically one by one, and the remainder of the sample is measured after
each absorption ; the difference in volume gives the partial volume occupied by the constituent in
the mixture.
Let us consider a volume V of a gaseous mixture at a temperature T, consisting of three
constituents A, B and C [Fig. 9.2 (a)]. Let us further assume that each of the constituents is com-
pressed to a pressure p equal to the total pressure of the mixture, and let the temperature remain
constant. The partial volumes then occupied by the constituents will be VA, VB and VC.

(a)(b)

Fig. 9.2

Now using the eqn. pV = mRT, we get

mA =

pV

RT

A

A

... referring to Fig. 9.2 (a)

and mA = pV

RT

A

A

... referring to Fig. 9.2 (b)

Now equating the two values for mA, we have

pV

RT

A

A

=

pV

RT

A

A

i.e., pAV = pVA

or VA = p

p

AV