Atomic mass: The atomic mass of an element is defined as the number of times an atom of that element is heavier than an atom of carbon-12. It is expressed in a.m.u. or atomic mass unit. In place of a.m.u., symbol u is used which is known as unified mass.
Atomic mass = Mass of an atom × 1 /12 mass of carbon atom

Molecular mass: Molecular mass of any substance is defined as the average relative mass of its molecules as compared to mass of an atom of carbon-12. Molecular mass is expressed as a.m.u. It is given by,
Molecular mass = Mass of one molecule of substance /(1/12) x mass of carbon -12. ,

Equivalent mass: Quantity of a substance in which is numerically equal to its equivalent mass is called its gram equivalent mass.
Calculation of Molarity:
Molar mass of K₂CO₃= 138 g/mol
Volume of solution = 500 mL or 0.5 L
Mass of  K₂CO₃ = 20.7
Number of moles = 20.7/138 = 0.149
Now,
Molarity = Number of moles/Volume of solution
= \(\frac{0.149}{0.5}\) = 0.299 M
Thus, molarity of solution is 0.299 M.