Question

A compound contains 20% of nitrogen by weight. If one molecule of the compound contains 2 nitrogen atoms, then the molecular weight (in $ g\text{ mo}{{\text{l}}^{-1}} $ )of the compound is:
(A) 35
(B) 70
(C) 140
(D) 280

Answer 1

Hint: The molecular mass and relative molecular mass are not the same as the molar mass, but they are linked. The molar mass is stated in g/mol and is defined as the mass of a particular substance divided by the quantity of that substance. When dealing with macroscopic (weighable) amounts of a material, the molar mass is typically the more suitable figure.

Complete answer:
The atomic masses of each nuclide present in the molecule are used to determine molecular masses, whereas the standard atomic weights of each element are used to derive molar masses. The isotopic distribution of the element in a given sample is taken into consideration by the standard atomic weight (usually assumed to be "normal"). Water, for instance, has a molar mass of 18.0153 g/mol.
By dividing the mass of a single element in one mole of a compound by the mass of one mole of the total compound, percent composition may be determined from a molecular formula. This number is shown as a percentage.
The chemical has two nitrogen atoms in each molecule.
As a result, 1 mol of chemical contains 2 mol of nitrogen atoms, resulting in 28 g of nitrogen.
However, the molecule has a nitrogen content of 20% by weight.
As a result, the molecule equivalent to 28 g of nitrogen weighs $ \dfrac{100\times 28}{20} $ =140 g.
As a result, the compound's molecular weight is 140 g/mol.
Hence option C is correct.

Note:
The total number of protons and neutrons (collectively known as nucleons) in an atomic nucleus is known as the mass number, also known as the atomic mass number or nucleon number. It is roughly equal to the atom's atomic mass (also known as isotopic mass) given in atomic mass units.