Question

What is equivalent mass? What is the number of equivalents? What is formula mass? What is formality?

Answer 1

These terms are very well used in chemistry or you can say these terms are the backbone of chemistry. We have used these terms in quantitative analysis to know the nature of atoms, molecules present. These terms are well used in experiments performed at labs.

Complete answer:
We will define each term one by one: Let’s start with equivalent mass first. The number of parts by mass of a substance that combines with or displaces $1.008$ parts by mass of hydrogen or $8.0$ parts of oxygen or $35.5$ parts of chlorine is called its equivalent mass (EM). Next move to Number of equivalent: It is calculated by dividing the molecular weight of solute by the number of equivalents per mole of solute. For example in case of acids, the number of equivalents per mole is the number of ${H^ + }\;$ions contributed by the acid per mole of acid. The formula mass of a substance is the sum of the average atomic masses of each atom represented in the chemical formula and is expressed in atomic mass units. Last one is Formality. The formality of a solution is defined as the number of formula mass of any solute dissolved in one litre of solution.

Note:
The formula mass of a covalent compound is also called the molecular mass. The equivalent mass is used to compare chemically different elements. Atoms combine with each other to form chemical compounds, such that the elements are always present in definite proportions by mass and this property can be used to make chemically different molecules with different mass.