Mole & Avagadro’s Constant
Relative Molecular Mass (Mr) is the sum of the relative atomic masses in a molecule e.g
C6H12O6 = 6 x 12 + 12 x 1 + 6 x 16 = 108.
The relative formula mass is used for ionic compounds in which the formula is an
empirical formula (the simplest ratio) e.g. NaCl 23 + 35.5 = 58.5. All have no units.
Find the Ar, Mr or relative formula mass for the following, identifying which it is.
a.
O
b.
O3
c.
MgO2
d.
C2H5Cl
e.
C12H22O12
f.
CO2 g.
H2SO4
h.
KmnO4
i.
Br2
j.
Mn
 As single atoms are so small and difficult to weigh, it is easier to take a lot of them
 The grouping used is the MOLE, which contains 6.023 x 1023 is known as
Avagadro’s constant (NA or L)
 Again this is based on the number of particles in 12 g of 12C
The mass of one mole of ANY substance is known as the MOLAR MASS (M). It has the
same numerical value as Ar, Mr and relative formula mass, but has the units g mol-1 or
g/mol. E.g. 1 mole of 12C has molar mass = 12 g mol-1.
What is the molar mass of:
a.
O3
b.
MgO2
c.
C12H22O12 d.
F2
e.
CH4
f.
H2CO3
g.
HCl
No. of mole is calculated using n = m = mass
M
molar mass
e.g. the no. of mole in 11 g of CO2
m = 11 g
M = 44 g mol-1
n = m/M = 11/44 = 0.25 mol
We can convert form no. mole to no. particles or vice versa using Avagadro’s constant.
no. particles = n x NA
e.g. no. CO2 molecules in 11 g CO2
n(CO2) = 0.25 mol
no. CO2 particles = 0.25 x 6.023 x 1023
= 1.5 x 1023 CO2molecules
no. O atoms in 11 g CO2 = 2 x no. CO2 molecules (as 2 O atoms in each molecule)
= 2 x 1.5 x 1023
= 3 x 1023