Mole
A mole of any substance is the amount of that
substance which contains many elementary particles as the number of atoms in 12
grams of carbon -12. These elementary particles can be atoms, ions, electrons,
molecules etc. A mole is a unit used in measuring the amount of particles.
The number of particles in one mole of any
substance is equal to 6.02 x 1023. This number is also known as
Avogadro’s number.
Molar
Mass
This is the mass of one mole of any substance
expressed in grams. It is also the formula mass which is the sum of all atomic
masses of all atoms in a compound expressed in grams.
Example question 1
Calculate the molar mass of the following
substances
i)
sodium chloride
ii)
chlorine
iii)
Oxygen
(Na=23, Cl =35.5, O=16
Solution
Sodium
chloride
The relative atomic mass of Na is 23 and that of
Chlorine is 35.5
The formula of sodium chloride is NaCl,
23 + 35.5 = 58.5
1 mole of sodium chloride contains 58.5g.
Chlorine
Chlorine is a diatomic molecule, Cl2
The relative atomic mass of chlorine is 35.5
Cl2 = 2 x35.5 = 71
1 mole of Chlorine contains 71g
Oxygen
Oxygen is also a diatomic molecule and its
relative atomic mass is 16
O2= 2 x 16 =32
1mole of oxygen contains 32g
Molar Volume
Molar volume is the volume occupied by one mole of
any gas at standard temperature and pressure (s.t.p). It is equal to 22.4dm3
or 22400cm3
22.4dm3 = 6.02 x1023 = 1mole
of any gas at s.t.p.
Example question 2
In a laboratory preparation of hydrogen, 5.4g of
potassium reacts with water as follows:
2K(s) + 2H2O(l)
------ 2KOH(aq) + H2(g)
Calculate:
a)
The mass of water which reacts with
5.4g of potassium
b)
The volume of hydrogen produced at
s.t.p
c)
The mass of hydroxide ions produced
(K=39, H=1, O=16, molar volume of gas =22.4dm3,
Avogadro’s number = 6.02x1023)
Solution
2K + 2H2O --------- 2KOH
+ H2
From the equation,
a.
2moles of K = 2moles of H2O
2 x 39g of K = 2 x18g of H2O
78g
of K = 36g of H2O
5.4g
of K = 36/78 x 5.4
=
2.49g
b.
2 moles of K = 1 mole of H2
78g of K = 22.4dm3 or
22400cm3
5.4g of K = 22400/78 x 5.4
= 1550 cm3
c.
2K + 2H2O ------- K+ +
2OH- + H2
2 moles of K = 2moles of OH-
ions
78g of K = 2 x 6.02x1023
5.4g of K = 1.204x1023/78x5.4
= 8.34x1022 of OH-
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