Question

QUESTION 2.3 (I)

Define the following term:

(i) Mole fraction

(ii) Molality

(iii) Molarity

(iv) Mass percentage

Answer 1

(i) Mole fraction may be defined as the ratio of the number of moles of one component (solute or solvent) to the total number of all the components present in the solution.
Mole fraction of a component
$=\frac{Numberofmolesofcomponent}{Totalnumberofmolesofallthecomponents}$
For example, in a binary mixture, if the number of moles of A and B are $n_A$ and $n_B$ respectively, the mole fraction of A and B will be $x_A$ and $x_B$. [Mole fraction is a ratio, so it has no unit.]
$x_A=\frac{n_A}{n_A+n_B};x_B=\frac{n_B}{n_A+n_B}$

(ii) The number of moles of the solute (n) per kilogram (kg) of the solvent is known as molality (m).
Molality $=\frac{Molesofsolute}{Massofsolventinkg}$
Unit of molality is $molk{g}^{-1}$ or m(molal)

(iii) Molarity Molarity (M) is defined as 'the number of moles of solute dissolved in one litre (or one cubic decimetre) of solution.
Molarity (M) $=\frac{Molesofsolute}{Volumeofsolutioninlitre}$
Unit of molarity is mol $L^{-1}$ or M (Molar)

(iv) Mass percentage It may be defined as mass of solute per 100 g of solution.
Mass $\%$ of solute $=\frac{Massofsolute}{Massofsolution}\times 100$