Calculations involving Gases
Chemical Ideas 1.4 deals with calculations involving gases.
Key terms in this chapter are:
Fixed mass of gas, Charles' Law Boyle's
Law.
Molar Volume.
Avogadro's Law.
Room Temperature and Pressure, r.t.p.
Standard Temperature and Pressure,
s.t.p.
In this section you are going to learn:
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That the volume of a fixed mass of gas depends upon its temperature and pressure.
More about the nature of temperature and pressure and the units used to measure
these.
That the volume of 1 mole of gas is called the molar volume.
The numerical values of the molar volume at r.t.p. and s.t.p.
How to do simple calculations involving volumes of gases reacting according to
balanced chemical equations.
If you take a fixed mass of gas, that is, a fixed number of molecules, its volume will depend
on the temperature and pressure at which it is measured.
1 mole of a gas is the mass of the Avogadro constant of molecules of that gas. The volume of
this is called the Molar Volume.
Element or
compound
Relative
Molecular Mass,
Mr
Mass of
1 mol
CO2
SO2
N2
H2
Cl2
CH4
Ar
NH3
44
64
28
2
71
16
40
17
44 g
64 g
28 g
2g
71 g
16 g
40 g
17 g
Each of the above masses of elements and compounds contains the same number of molecules and
occupies the same volume when measured at the same temperature and pressure.
Avogadro's Law says that equal volumes of all gases contain the same number of molecules
when measured at the same temperature and pressure.
There are two sets of conditions of temperature and pressure used in chemistry:
Room Temperature and Pressure
-2
25 °C (298 K) and 1 atmosphere (100 000 Nm or Pa)
Standard Temperature and Pressure
-2
0 °C (273 K) and 1 atmosphere (100 000 Nm or Pa)
The molar volume will, of course, be different for each of these:
3
Molar Volume at r.t.p. is 24 dm .
3
Molar Volume at s.t.p. is 22.4 dm .
Q1.
Calculate the mass of carbon dioxide gas, CO2, that would occupy a
volume of 18 dm3 when measure at room temperature and pressure.
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You may recall that a balanced chemical equation gives the amounts of reactants that react
and the amounts of products that form in moles.
Consider the reaction below:
N2(g) + 3H2(g) →
2NH3(g)
This equation tells us that 1 mole of N2(g) reacts with 3 moles of H2(g) to form 2 moles of
NH3(g). Therefore, at room temperature and pressure, 24 dm3 of N2(g) reacts with 72 dm3 of
H2(g) to form 48 dm3 of NH3(g). Put more generally, 1 volume of N2(g) reacts with 3
volumes of H2(g) to form 2 volumes of NH3(g).
Q2.
What volume of oxygen is required to react completely with a mixture of 10
cm3 of hydrogen and 20 cm3 of carbon monoxide?
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