Write about Molarity and Molality.

(N/A) Molarity: It is the most widely used unit and is denoted by $M$.
It is defined as the number of moles of the solute in $1 \ L$ of the solution.
$\text{Molarity } (M) = \frac{\text{No. of moles of solute}}{\text{Volume of solution in litres}}$
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 $1 \ L$ of the solution. For $0.2 \ M$ solution,we require $0.2 \ mol$ of $NaOH$ in $1 \ L$ solution.
For the calculation of molarity of two solutions,the following formula is used:
$M_1 \times V_1 = M_2 \times V_2$
Molality: It is defined as the number of moles of solute present in $1 \ kg$ of solvent. It is denoted by $m$.
$\text{Molality } (m) = \frac{\text{No. of moles of solute}}{\text{Mass of solvent in } kg}$