Question

Explain molarity, molality, normality, formality and mole fraction with definition and example.

Answer 1

Hint: As a general idea Molarity is moles per litre, Molality is moles per kilogram, Normality is normals per liter, Formality is number of formula mass per litre, Mole fraction is ratio of moles present, These details should be kept in mind when approaching on these stoichiometric quantities.

Complete step-by-step answer:
Molarity of a substance is defined as the number of moles of a substance dissolved per litre of the solution, here moles defines the amount of a substance containing Avogadro’s number of constituents in it which has a numeric value of $6.022 \times {10^{23}}$.

Molality is defined as the number of moles of substance dissolved per kilogram of the solvent, a word of caution is to note that molality talks about solute and solvent relation and not about solute and solution relation.

Normality is defined as the number of gram equivalents dissolved per litre of solution, One gram equivalent is defined as the number of moles of substance divided by the n-factor, n-factor represents the active role of the species during the reaction.

Formality is defined as the number of formula units dissolved per litre of solution, Formula units represents chemical formula of a chemical species which may or maynot even exist in solution in that form for example $KI$exists as ${K^ + }$ and ${I^ - }$ in solution but the formula units would count one potassium and one iodide as Potassium iodide.

Mole fraction is the ratio of moles of a substance to the total number of moles in the solution, This is a unit less quantity and gives a generic idea on the relation between various species in terms of stoichiometry.

Additional Information: Molality is measured in terms of solvent whereas every other quantity has dependence on the whole of solution. Mole fraction is unit less unlike other quantities.

Note: Depending on the type of problem provided or the details easily available in a laboratory while performing a reaction we use above stoichiometric quantities, In general Molarity, Normality and Mole fraction suffice almost all the cases we come across however sometimes we also come across molality.