Define atomic mass, molecular mass and equivalent weight. In 500 mL solution, 20.7g potassium carbonate is dissolved. Calculate its molarity. (molecular of ${K_2}C{O_3}$= 138).

Hint: The molarity of the solution is a method to calculate the concentration of the solute in terms of mole per liter. To calculate molarity the first number of moles is determined by dividing mass by the molecular mass.

Complete step by step answer:
The atomic mass is defined as the mass of a single atom of a chemical element which includes three subatomic particles which are present in the atom. The subatomic particles are protons, neutrons and electrons.
The molecular mass is defined as the total mass of the molecule which is calculated by summing the individual atomic mass of the atom. The number of atoms is multiplied with the atomic mass.
The equivalent weight is defined as a mass of compound present grams which is equivalent to eight grams of oxygen atom or one gram of hydrogen atom.
Given,
Volume of the solution is 500 mL.
Mass of potassium carbonate is 20.7 g.
Molecular weight of ${K_2}C{O_3}$ is 138
The formula for calculating the number of moles is shown below.
$n = \dfrac{m}{M}$
Where,
n is the number of moles.
m is the given mass
M is the molecular weight
To calculate the moles of potassium carbonate, substitute the values of mass and molar mass in the above expression.
$\Rightarrow n = \dfrac{{20.7g}}{{138g/mol}}$
$\Rightarrow n = 0.15mol$
The molarity of the solution is defined as the moles of solute dissolved in one liter solution.
The formula to calculate molarity is shown below.
$M = \dfrac{n}{V}$
Where,
M is the molarity
n is number of moles
V is volume
To calculate the molarity, substitute the values in the above equation.
$\Rightarrow M = \dfrac{{0.15}}{{0.5}}$
$\Rightarrow M = 0.3M$

Note: In the given formula of molarity, the value of volume is substituted 0.5 instead of 500. This is because the molarity is calculated in terms of mole per litre, therefore 500 mL is converted into litre.