Q.A sample of 12 M Concentrated hydrochloric acid has a density 1.2 gL$^{-1}$. Calculate the molality.

Concentration expresses the amount of solute present in a given quantity of solvent or solution. Different situations call for different units, chosen for calculation convenience:

• Molality (m) $=\dfrac{\text{moles of solute}}{\text{mass of solvent in kg}}$ -- independent of temperature (defined by mass, not volume).
• Molarity (M) $=\dfrac{\text{moles of solute}}{\text{volume of solution in L}}$ -- used for 1:1 mole-ratio titrations (e.g. EDTA complexometric titrations); temperature-dependent, since solution volume changes slightly with temperature.
• Normality (N) $=\dfrac{\text{gram equivalents of solute}}{\text{volume of solution in L}}$ -- used in redox and acid-base (neutralisation) titrations.
• Formality (F) $=\dfrac{\text{formula weights of solute}}{\text{volume of solution in L}}$ -- used for ionic compounds without discrete molecules.
• Mole fraction (x) $=\dfrac{\text{moles of one component}}{\text{total moles of all components}}$ (always $x_A+x_B=1$ for a binary mixture) -- independent of temperature; used to calculate partial pressures of gases and vapour pressures of solutions. …