If $5.85 \ g$ of $NaCl$ is dissolved in $72 \ g$ of water,find the mole fraction of $NaCl$.

The molarity of the solution obtained by mixing $750 \ mL$ of $0.5 \ (N) \ HCl$ with $250 \ mL$ of $2 \ (M) \ HCl$ is ........... $M$.

The amount of ammonium sulphate required for the preparation of $1 \, M$ solution in $2 \, L$ is ............ $gm$.

State whether the following sentences are true $(T)$ or false $(F)$.
$(a)$ Concentration of pure water is $1.0 \ mol \ L^{-1}$.
$(b)$ Concentration of pure water is $55.5 \ mol \ L^{-1}$.

Calculate the molarity of a solution of ethanol in water in which the mole fraction of ethanol is $0.040$ (assume the density of water to be $1 \, g/mL$).

Air is an example of a solution of