Question

Define the following terms:
(i) Mole fraction
(ii) Molality
(iii) Molarity
(iv) Mass percentage.

Answer 1

(i) Mole Fraction

Definition:
Mole fraction is a way of expressing the concentration of a component in a mixture. It is defined as the ratio of the number of moles of one component to the total number of moles of all components in the mixture.

Formula:
$\text{Mole Fraction (X)} = \frac{\text{Number of moles of component}}{\text{Total number of moles of all components}}$

For a solution with components A and B:
$X_A = \frac{n_A}{n_A + n_B}$
$X_B = \frac{n_B}{n_A + n_B}$

where $n_A$ and $n_B$ are the number of moles of components A and B, respectively.

(ii) Molality

Definition:
Molality is a measure of the concentration of a solute in a solution. It is defined as the number of moles of solute per kilogram of solvent.

Formula:
$\text{Molality (m)} = \frac{\text{Number of moles of solute}}{\text{Mass of solvent in kilograms}}$

For example, if 2 moles of solute are dissolved in 1 kilogram of solvent:
$m = \frac{2 \text{ moles}}{1 \text{ kg}} = 2 \text{ mol/kg}$

(iii) Molarity

Definition:
Molarity is another measure of concentration, defined as the number of moles of solute per liter of solution.

Formula:
$\text{Molarity (M)} = \frac{\text{Number of moles of solute}}{\text{Volume of solution in liters}}$

For example, if 1 mole of solute is dissolved to make 1 liter of solution:
$M = \frac{1 \text{ mole}}{1 \text{ liter}} = 1 \text{ M}$

(iv) Mass Percentage

Definition:
Mass percentage is a way of expressing the concentration of an element or component in a mixture. It is defined as the mass of the component divided by the total mass of the mixture, multiplied by 100%.

Formula:
$\text{Mass Percentage} = \left( \frac{\text{Mass of component}}{\text{Total mass of mixture}} \right) \times 100\%$

For example, if a solution contains 5 grams of solute in 95 grams of solvent, the mass percentage of the solute is:
$\text{Mass Percentage} = \left( \frac{5 \text{ g}}{5 \text{ g} + 95 \text{ g}} \right) \times 100\% = \left( \frac{5}{100} \right) \times 100\% = 5\%$

These definitions provide a clear understanding of different ways to express concentrations of solutions in chemistry.