Formula Mass
For ionic compounds, such as sodium chloride, there is no
discrete individual molecule. Instead, these are the aggregates represented by
a formula of the type, (A+ B–)n.
Solid sodium chloride is represented by the formula (Na+Cl–)n,or (NaCl)n. Thus, the simplest formula
for sodium chloride can be written as NaCl.
So, the concept of formula mass is used for ionic compounds.
The sum of the atomic masses of
all atoms in a formula unit of a substance is called the formula mass of the
substance.
Calculating the formula mass of an ionic substance
The formula mass of any ionic compound is expressed as the sum
of the atomic masses of all the atoms present in its formula.
For example, for NaCl,
Formula mass of NaCl = (1 × Atomic mass of Na) + (1 × Atomic
mass of Cl)
We know that
Atomic mass of Na = 23 u and Atomic
mass of Cl = 35.5 u
So, Formula mass of NaCl = (1 × 23 u) + (1 × 35.5 u) = (23 +
35.5) u = 58.5 u
Thus, the molecular mass (or empirical formula mass) of a
substance is equal to the sum of the atomic masses of all the elements present
in its molecular (or empirical) formula, taking into account the number of
atoms of each element present in its molecular (or empirical) formula.
For example,
Molecular mass of CO2
1 C =
1 × 12 u = 12 u
2 O = 2 × 16 u = 32 u
Molecular mass of CO2 = 44 u
Formula mass of Al2O3
2Al = 2 × 27 u = 54 u
3 O = 3 × 16 u = 48 u
Formula mass of Al2O3 =
102 u
Formula mass of Na2SO4.10H2O
2 Na = 2 × 23 u = 46 u
1 S = 1 × 32 u = 32 u
4 O = 4 × 16 u = 64 u
20 H = 20 × 1 u = 20 u
10 O = 10 × 16 u = 160 u
Formula mass of Na2SO4.10H2O
= 322 u
Formula mass of K2Cr2O7
2 K = 2 × 39 u = 78 u
2 Cr = 2 × 52 u = 104 u
7 O = 7 × l6 u = 112 u
Formula mass of K2Cr2O7
= 294 u
The concept of formula mass is
applicable only to the ionic substances.
Do It
Yourself
Calculate
the formula mass of:
- KBr
- Na2S
- Ca(OH)2
- MgCl2
- NH4NO3
- Cu(NO3)2
- Na2CO3.10H2O
- C6H12O6
- Fe2O3
- (NH4)2SO4
- P2O5
- CaSO4
