CBSE Class 11-science Answered

Formula Mass

• Mass of a molecule of an ionic compound.
• It is  the sum of atomic masses of all the elements present in one formula unit of a compound.
• It is used for measuring the molecular mass of entities which do not exist in the solid form.

• Substances such as sodium chloride do not contain discrete molecules as their constituent units. In such compounds, positive (sodium) and negative (chloride) entities are arranged in a three-dimensional structure.

• In sodium chloride, one Na⁺ is surrounded by six Cl⁻ and vice versa.

Hence, in such cases, the formula mass is used to calculate the mass instead of the molecular mass. Just like the molecular mass, it is calculated by adding the atomic masses of the atoms present in one formula unit.

• Thus, formula mass of sodium chloride = atomic mass of sodium + atomic mass of chlorine

                                                                     = 23.0 u + 35.5 u

                                                                     = 58.5 u