Molarity is the most widely used unit and is denoted by M. It is defined as the number of moles of the solute in 1 litre of the solution.
Thus, Molarity (M) = Number of moles / Volume of Solution
Suppose we have 1 M solution of a substance, say NaOH and we want to prepare a 0.2 M solution from it.
1 M NaOH means 1 mol of NaOH present in 1 litre of the solution. For 0.2 M solution we require 0.2 moles of NaOH in 1 litre solution.
Hence, we have to take 0.2 moles of NaOH and make the solution to 1 litre.
Now how much volume of concentrated (1M) NaOH solution be taken which contains 0.2 moles of NaOH can be calculated as follows:
If 1 mol is present in 1 L or 1000 mL, then 0.2 mol is present in
1000 mL/1Mol x 0.2 = 200 mL
Thus, 200 mL of 1M NaOH are taken and enough water is added to dilute it to make it 1 litre.