Question

What are molarity, molality, and normality? Explain with an example.

Answer 1

Molarity

• Molarity of solution is a weight by volume relationship to represent its strength and is defined as 'the number of moles of solute dissolved in one liter of solution.

Formula of molarity

$\text{Molarity=}\frac{\text{Moles of solute}}{\text{Volume of solution in litre}}$

Example of molarity

• $1.25{\text{mol L}}^{-1}$or $\left(1.25\text{M}\right)$ solution of $\text{KOH}$ means that $1.25\text{mol}$ of $\text{KOH}$ has been dissolved in one liter (or one cubic decimetre).

Molality

• Molality (m) is defined as the number of moles of the solute per kilogram (kg) of the solvent.

Formula for molality

$\text{Molality=}\frac{\text{Moles of solute}}{\text{Mass of solvent in kg}}$

Example of molality

• $1{\text{mol kg}}^{-1}$$\left(\mathrm{or}1\mathrm{m}\right)$ solution of $\mathrm{KCl}$ means that $1\text{mol}$$(74.5\text{g})$ of $\mathrm{KCl}$ is dissolved in $1\text{kg}$ of water.

Normality

• It is the measure of concentration equal to the gram equivalent mass of a substance per liter of solution.

Formula for normality

$\text{Normality=}\frac{\mathrm{number}\mathrm{of}\mathrm{gram}\mathrm{equivalents}\mathrm{of}\mathrm{solute}}{\mathrm{Volume}\mathrm{of}\mathrm{solvent}\mathrm{in}\mathrm{Litre}}$

Example of normality

• $1\text{N}$ $\text{HCl}$ solution is one Molar $\text{HCl}$solution because only $1$ mole of ${\mathrm{H}}^{+}$ is formed per mole of $\text{HCl}$.