Atomic Mass

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 mass.

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 M.F)

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 Molecular formula.

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 molecule.

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, (CH2O).

2.2.1 (b) 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 glucose n=6.

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).

2.2.2 Molecular Formula Mass:

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 a.m.u, respectively.

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.

For example, 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.

2.2.3 Formula Mass:

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 ionic compounds.

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.

For example, 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.

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