How to find Molecular Mass

Atom and Molecule of Class 9

The molecular mass of a substance may be defined as the relative mass of its molecule as compared to the mass of an atom of carbon (carbon - 12) taken as 12 units.

Molecular mass expresses as to how many times a molecular of a substance is heavier than 1/12th of the mass of an atom of carbon (carbon -12).

Eg. A molecule of water is 18 times heaver than 1/12th of the mass of carbon atom. Therefore, the molecular mass of water is 18u. Similarly, the molecular mass of CO2 is 44u.

Calculation : The molecular mass is equal to sum of the atomic masses of all the atoms present in one molecule of the substance.

Eg. The molecular mass of Sulphuric Acid (H2SO4) can be calculated as follows :

Molecular mass of H2SO4 = Mass of 2 H atoms + Mass of 1 S atom + Mass of 4 O atoms

  = (2 × 1) + (1 × 32) + (4 × 16) = 2 + 32 + 64 = 94u.

Thus, the molecular mass of Sulphuric acid is 98u.

Atoms are the components of molecules and the molecules are the components of elements or compounds.

WHAT IS AN ION?

The molecular compounds composed of metals and non-metals contain charged species. The charged species are known as ions. Ions can be defined as a positively or negatively charged atom (or group of atoms). Depending upon the charge they carry, ions can be of two types :

(a) Cation:

If an atom has less electrons than a neutral atom, then it gets positively charged and a positively charged ion is known as cation.

e.g. Sodium ion (Na+), Magnesium ion (Mg2+) etc.

A cation bears that much units of positive charge as are the number of electrons lost by the neutral atom to form that cation.

e.g. An aluminum atom loses 3 electrons to form aluminum ion, so aluminum  ion bears 3 units of positive charge and it is represented as Al3+.

All metal elements form cations.

(b) Anion:

If an atom has more number of electrons than that of neutral atom, then it gets negatively charged and a negatively charged iron is known as union.

e.g. Chloride ion (Cl-), oxide ion (O2-) etc.

All non-metal elements form anions (except hydrogen).

An anion bears that much units of negative charge as are the number of electrons gained by the neutral atom to form that anion.

e.g. A nitrogen atom gains 3 electrons to form nitride ion, so nitride ion bears 3 units of negative charge and it is represented as N3-.

Size of a cation is always smaller and anion is always greater than that of the corresponding neutral atom.

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