CBSE Class 11-science Answered

could you plz explain more about normality and formality in detail with suitable examples?

Asked by SIVENDU SIVENDU | 19 Jul, 2012, 05:07: PM

Expert Answer

It is defines as the number of gram equivalents of solute present per litre of solution. It is denoted by N.

Normality (N) = Number of gram equivalents of solute/Number of litres of the solution

or Normality × Number of llitres of the solution

= Number of gram equivalents of the solute

For an acid, the chemical equivalent is the number of moles of H⁺¹ ion. For a base, the chemical equivalent is the number of moles of OH^-1ions. For an oxidation-reduction solution, the chemical equivalent is the number of moles of electrons transferred.

Formality, F, is the number of formula weight units of solute per liter of solution. Remember that one mole of a compound has a mass equal to the formula weight in grams. The number of formula weight units is equal to the number of moles for molecular substances.

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 isnt 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 doesnt 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.

Answered by | 19 Jul, 2012, 07:18: PM