Question

Define atomic mass, molecular mass, and equivalent weight. In 500 mL solution, $20.7$g of potassium carbonate is dissolved. Calculate its molarity. (Molecular weight of potassium carbonate= 138)

Answer 1

The molarity of a solution can be defined as the number of moles of a substance dissolved in one litre of the solution.

Formula: ${\text{molarity}} = \dfrac{{{\text{moles}}}}{{{\text{volume of solution}}}}$

Complete Step by step Solution:
- The atomic mass of an element can be defined as the mass of on atom of the element, while the relative atomic mass or the atomic weight can be defined as the mass present in ${{\dfrac{1}{12}}^{\text{th}}}$ the mass of a carbon-12 isotope.
- The molecular mass can be defined as the mass present in 1 molecule of a substance. The molecular weight can be defined as the sum of the atomic weights of the constituent atoms of the molecule.
- 1 mole of a substance is equal to the atomic weight of an element and in case of the compounds, it is equal to the molecular weight of the substance.
- The equivalent weight of a substance is defined as the molecular weight of a substance divided by the valence of the substance.
- The molarity of a solution is mathematically expressed as, M = $\dfrac{n}{v}$, where n is the number of moles and v is the volume.
No. of moles, n = $\dfrac{20.7}{138}$ = $0.15$ moles in 500 mL.
Therefore, the molarity of the solution is $0.30$.

Notes: The number of gram equivalents of a substance in one litre of the solvent is called the normality of the solution, while the amount of a substance present in 1 kg of the solvent is called the molality of the solution.