2.2 Atomic Mass
The mass of an atom depends upon the number of protons
and neutrons present in it. As the atoms are extremely
small particles, it is difficult to weigh them directly.
For example the mass of single hydrogen (H) atom, is
1.6x10-24g (0.000 000 000 000 000 000 000
0016g). Clearly we cannot weigh a hydrogen atom or any
other kind of atom, by placing it
on a balance pan. Scientist needs special method to
obtain the mass of an atom by comparison to a standard
In 1961, by an international agreement, an atom of C-12,
that has 6 protons and 6 neutrons has a mass of exactly
12 atomic mass units (a.m.u.) taken as a standard. So
one atomic mass unit (1 a.m.u.) is defined as a exactly
equal to one-twelfth the mass of C-12 atom. Since most
Sterner s consists of a mixture of isotopes (Isotopes
are the atoms of same clement, having same atomic number
but different atomic masses). For example, naturally
occurring carbon is composed of 98.889 percent C-12 and
111 percent C-13. Thus average atomic mass of C-atom
becomes 12.014 a.m.u.
The atomic mass of an element is now taken as, the
average mass of natural mixture of isotopes which is
compared to the mass of one atom of C-12 a.m.u
Thus the atomic mass of oxygen (0) = 16 a.m.u and that
of Sulphur(S) = 32 a.m.u.
2.2.1 Empirical Formula and Molecular Formula: (E.F and
A formula is a combination of symbols for atoms or ions,
that are held together chemically in a compound. By
formula we mean not only the elements present but also
ratios in which the atoms are combined. Hence we will
discuss two types of formulas i.e. Empirical formula and
2.2.1 (a) Empirical Formula (E.F): (Simplest formula).
A formula that gives only the relative number of each
type of atoms present in a molecule. In other words, the
empirical formula does not necessarily give the actual
number of atoms in a molecule. For example, the
molecular formula of benzene is C6H6.
This formula indicates that benzene molecule consists of
(6) carbon atoms and (6) hydrogen atoms. The ratio of
carbon (C) to hydrogen (H) atoms in this molecule is 6:6
or 1:1 the empirical formula of benzene is, therefore
written as (CH).
Thus empirical formula tells us "which elements are
present and their simplest atomic ratio, but not
necessarily the actual number of atoms present in the
Consider another example, the molecule of glucose (C6H12O6)
in which the ratio of C, H and O atoms is 6:12:6 i.e.
1:2:1. The empirical formula of glucose is, therefore,
Molecular Formula (M. F):
Molecular formula indicates the actual number and type
of atoms in a molecule. It can either be same as
empirical formula or some simple multiple of it.
Mathematically, M. F = (E.F) n, where (n) is the whole
number. For example the molecule of CO2
consists of one atom of carbon in combination with two
atoms of oxygen. The formula (CO2) is the
molecular formula of carbon dioxide. It represents the
true composition- of a molecule of the compound. The
molecular formula may be same as empirical formula as in
the case of CO, or some simple multiple of empirical
formula. Thus the molecule of glucose which shows that
the molecule of glucose, consists of (6) carbon, (12)
hydrogen and (6) oxygen atoms and its simplest atomic
ratio i.e. empirical formula is (CH2O). Thus
the molecular formula of glucose (C6H12O6)
is equal to (CH2O)6 or six times
to empirical formula (CH2O).
It follows that molecular formula of glucose is six
times the empirical formula, which is obtained by M. F =
(E. F)n where (n) is the whole number, and in
Mathematically, n= M.F.weight
For many molecules, the molecular formula and empirical
formula are the same, some examples are formaldehyde (CH2O),
ammonia (NH3) and methane (CH4).
The molecular formula mass (molecular mass) of a
substance is the sum of the atomic masses-of all atoms
present in the molecular formula of a substance or
molecule. Taking as an example let us calculate the
molecular formula mass of CO2 the molecule of
CO2 contains one atom of C and two atoms of
O. The atomic masses of C and O are 12 a.m.u. and 16
C =12 x 1 = 12 a.m.u.
O =16 x 2 = 32 a.m.u.
Molecular formula mass of CO2
= 44 a.m.u.
compute the molecular formula mass of ozone (O3).
Molecular formula mass is calculated by adding together
the atomic masses of the constituent atoms. The ozone (O3)
molecule contains three oxygen atoms
Each of which has mass equal to 16a.m.u.
Therefore molecular formula mass of ozone (O3)
= 3 x 16 = 48 a.m.u.
Formula mass of substances is the sum of the atomic
masses of all atoms in a formula unit of the substance.
For example, we can calculate the formula mass of sodium
chloride (NaCl), a common salt, by adding the atomic
masses of all atoms in the formula unit, expressed in (a.m.u.).
The atomic masses of (Na) and (Cl) are 23 a.m.u. and
35.5 a.m.u., respectively.
Na = 23 x 1 = 23 a.m.u.
CI = 35.5 x 1 = 35.5 a.m.u.
Formula mass of NaCl = 58.5 a.m.u.
That the term molecular mass applies to molecular
compounds. The term formula mass can be used with
either molecular compounds or ionic compounds. The
term molecular mass cannot be used with ionic
compounds because there are no discrete molecules in
2.2.4 Molar Mass:
Molar mass of a substance is its relative molecular mass
expressed in grams. Thus molar mass of a substance has a
fixed unit. For example, 1 mole of (C) is equal to its
atomic mass expressed in grams.
Molecular mass of C = 12 a.m.u. and therefore the molar
mass of carbon would be 12g.
calculate the molar mass of ammonia (NH3).
The molar mass is obtained by adding the atomic masses
of component atoms.
N = 1 x 14g = 14g
H = 3 x 1g = 3g
Molar mass of NH3 = 17g
Remember, that relative molecular mass of NH3 * 17 a.m.u.