CBSE Class 10 Answered

Normality (N) is the number of gram equivalents of the solute dissolved per litre of the solution.

Normality = number of gram equivalents of the solute / Volume of solution in litres

Formality (F) is the number of formula masses of the solute dissolved per litre of the solution.

Formality = number of formula masses of the solute / Volume of solution in litres

Remember that one mole of a compound has a mass equal to the formula mass in grams. The number of formula mass units is equal to the number of moles for molecular substances. The purpose of formality is to distinguish the number of moles of a compound from the number of moles of ions in solutions of ionic compounds or weak electrolytes. If we dissolve one mole of calcium nitrate in enough water to make a liter of solution, the formality is one. The molarity (as it is commonly used) of calcium nitrate is the same as the formality, but that is a little sloppy because, once dissolved, the calcium nitrate ionizes completely so there isn’t really any Ca(NO₃)₂ in the solution. The molarity of the nitrate ions is two molar, because one formula unit has two nitrate ions. The formality of nitrate ion doesn’t exist because it is defined as the number of moles of the entire compound. So the only difference between formality and molarity is that you can express the molarity of the different ions individually and the formality is the entire compound irrespective of ionization. Formality is somewhat old fashioned. Modern more casual usage is to use molarity for both ideas and label with the appropriate solute. For example 1.0 F solution of Ca(NO₃)₂ is 2.0 M NO₃^-1 or 1.0 M Ca(NO₃)₂.

Normality is related to molarity as N = nM

Where N is normality, M is molarity, n is number of moles.

Formaltiy is used for ionic compounds whereas normality is used for acids and bases & oxidising and reducing agents.