Question

Define the following terms:
a) Mole fraction
b) Molality
c) Molarity
d) Mass percentage

Answer 1

A mole is a unit of measurement for the amount of substance. Moles give us a consistent method to convert between atoms/molecules and grams.

Complete Solution:
Mole fraction: The mole fraction of a component in a mixture is defined as the ratio of the number of moles of the component to the total number of moles of all the components in the mixture.
i.e., the Mole fraction of a component =$\dfrac{number\text{ of moles of component}}{total\text{ number of moles of the components}}$
Mole fraction is denoted by ‘x’:
If in a binary solution, the number of moles of the solute and solvent is respectively:
${{x}_{A}}=\dfrac{{{n}_{A}}}{{{n}_{A}}+{{n}_{B}}}$
Similarly, the mole fraction of the solvent in the solution is given as;
${{x}_{B}}=\dfrac{{{n}_{B}}}{{{n}_{A}}+{{n}_{B}}}$
where ${{x}_{A}}\text{ and }{{\text{x}}_{B}}$ = mole fraction of A and B respectively.

Molality: Molality (m) is defined as the number of moles of the solute per kilogram of the solvent.
It is expressed as:
$molality=\dfrac{\text{moles of solute}}{moles\text{ of solvent per kg}}$

Molarity: Molarity (M) is defined as the number of moles of the solute dissolved in one Litre of the solution.
It is expressed as:
$molarity=\dfrac{\text{moles of solute}}{volume\text{ of solution in litre}}$

Mass percentage: The mass percentage of a component of a solution is defined as the mass of the solute in grams present in 100 g of the solution.
It is expressed as:
$\text{mass percent=}\dfrac{\text{mass of components in solutions}}{total\text{ mass of solution}}\times 100$

Note: The possibility to make a mistake is that you may confuse between molarity and molality. Molarity is the number of moles of solute in 1L of the solution while molality is moles of solute in the 1 Kg of the solvent.