|
The
mole concept - Avogadro's
Law and molar volume
Francine
Taylor-Campbell, Contributor
 |
|
The
All Da Costa Cup Football team
battles the All Schools Team
from Trinidad and Tobago. The
All DaCosta team won 3-1. -
Ian Allen Photo
|
POINTS
TO NOTE
Avogadro's Law states that equal volumes
of all gases measured at the same
temperature and pressure contain equal
numbers of molecules.
The temperatures and pressures usually
used are standard temperature and pressure
(STP), which is 0°C and 1 atmosphere
pressure and room temperature (RTP)
which is 20°C and 1 atmosphere pressure.
1
dm³ of any gas at STP contains
the same number of molecules as 1
dm3 of any other gas at STP.
22.4
dm³ of any gas contains L molecules
(6.0 * 10²³) at STP. This
is one mole of the gas and is called
the molar volume.
At
RTP the molar volume is 24 dm³.
CALCULATIONS
ON MOLAR VOLUME
1(a)
What is the volume of 8g of oxygen
at (i)RTP (ii) STP
1
mole of oxygen (O2)
= 2 * 16 = 32g
32g
= 24dm³ at RTP and 22.4 dm³
at
STP
Thus
8g = 24/4 dm³ at RTP and 22.4/4
dm³ at STP
Volume
of 8g of O2 at RTP = 6 dm³ and
at STP = 5.6 dm³
(b)
Calculate the mass of 4.8 dm³
of (i) N2 (ii)
CO2 at RTP.
(i)
1 mole of N2
= 28g
24
dm³ at RTP = 28g
4.8
dm³ = (28 * 4.8)/24 = 5.6g
(ii)
4.8 dm3 of CO2 at RTP = (44 * 4.8)/24
= 8.8g
2.
This question is concerned with the
mole and its application.
(a)
Define the following:
(i)
Avogadro's law
(ii)
The mole
(iii) The relative molar mass (6
marks)
(b)
Determine
(i)
the number of moles of carbon in 18g
of carbon (Relative atomic mass: C
= 12)
(ii)
the number of atoms in 18g of carbon
(iii)
the number of molecules in 4.9g of
sulphuric acid (RMM of sulphuric acid
= 98) (3 marks)
(c)
At room temperature and atmospheric
pressure (rtp), 1.6g of methane occupy
a volume of 2.4dm³. Calculate
the relative molar mass of methane.
(ONE mole of any gas at rtp has a
volume of 24 dm³) (2 marks)
ANSWERS
2.
(a) (i) Avogadro's Law states that
equal volumes of gases measured at
the same temperature and pressure
contain equal number of molecules.
(ii)
The mole is the amount of substance
that contains Avogadro's number of
particles, that is 6.0 * 10²³
particles.
(iii)
The relative molar mass is the mass
of one mole of a substance compared
with the mass of one atom of carbon-12.
(b)
(i) Number of moles of carbon == 1.5
mol (1 mol of carbon has a mass of
12g. Therefore # mol in 18g == 18g/12
gmol-1 = 1.5 kjmol)
(ii)
18 g of carbon == 1.5 mol
Now
1 mol has 6.0 * 10²³ atoms,
thus 1.5 mol has 1.5 x 6.0 * 10²³
atoms
==
9 * 10²³ atoms.
(iii)
# mol in 4.9g of sulphuric acid (H2SO4)
= 4.9/98 == 0.05 mol
Number
of molecules of H2SO4
== 0.05 x 6.0 * 10²³ = 3*10²²
molecules.
(c)
1 mole CH4 has
volume of 24dm³ at r.t.p.
Volume
of gas = 2.4 dm³ thus # mol =
2.4/24 = 0.1 mol CH4
0.1
mol CH4 has
a mass of 1.6g
Molar
mass or mass of 1 mol = 1.6/0.1 ==
16g/mol
*
Francine Taylor-Campbell is
an independent contributor.
|
