Question

b) What is formula unit of mass? How is it different from molecular mass​

Answer 1

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

Answer 2

Answer:

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The mass of the formula unit of a substance is the sum of the atomic masses of all atoms in an ionic compound's formula unit. The trick to measuring an ionic compound's formula mass is to accurately count each atom in the formula and multiply its atoms' atomic masses accordingly.

Formula mass is defined as the sum of atomic masses of the ions present in the formula unit of an ionic compound whereas molecular mass is the sum of the atomic masses of the atoms in a molecule.