Page 53 - Chemical engineering design

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CHEMICAL ENGINEERING
Solution
Basis of calculation 100 kg of 28 per cent w/w acid.
Molecular mass: water 18, HCl 36.5
Mass HCl D 100 ð 0.28 D 28 kg
Mass water D 100 ð 0.72 D 72 kg
28
kmol HCl D D 0.77
36.5
72
kmol water D D 4.00
18
Total mols D 4.77
0.77
mol fraction HCl D D 0.16
4.77
4.00
mol fraction water D D 0.84
4.77
Check total 1.00

Within the accuracy needed for technical calculations, volume fractions can be taken
as equivalent to mol fractions for gases, up to moderate pressures (say 25 bar).
Trace quantities are often expressed as parts per million (ppm). The basis, weight or
volume, needs to be stated.
quantity of component 6
ppm D ð 10
total quantity
Note. 1 ppm D 10 4 per cent.
Minute quantities are sometimes quoted in ppb, parts per billion. Care is needed here,
9
12
as the billion is usually an American billion (10 ), not the UK billion (10 ).

2.5. STOICHIOMETRY
Stoichiometry (from the Greek stoikeion element) is the practical application of the
law of multiple proportions. The stoichiometric equation for a chemical reaction states
unambiguously the number of molecules of the reactants and products that take part; from
which the quantities can be calculated. The equation must balance.
With simple reactions it is usually possible to balance the stoichiometric equation by
inspection, or by trial and error calculations. If difficulty is experienced in balancing
complex equations, the problem can always be solved by writing a balance for each
element present. The procedure is illustrated in Example 2.3.

Example 2.3

Write out and balance the overall equation for the manufacture of vinyl chloride from
ethylene, chlorine and oxygen.