Question

Question:
The number of moles of 7g of nitrogen gas is:
A.0.25
B.0.5
C.0.75
D.1

Answer 1

Hint: The number of moles for a substance correspond directly to the molecular mass of the given substance. In simpler words, if the molecular of a given compound is ‘x’ grams, then the number of moles present in ‘x’ grams of that solid is 1 mole and the number of atoms / molecules present is equivalent to Avogadro’s number.

Complete step by step answer:
The atomic number of nitrogen is 7 while the atomic mass number of nitrogen is 14. The atomic mass number also represents the weight of 1 mole of sample of the given substance. This means that the atomic mass of one mole of nitrogen is 14 grams. Also, nitrogen gas exists in the form of a bimolecular structure. Hence the molecular mass of nitrogen is \[\left( {14{\text{ }}x{\text{ }}2} \right) = 28{\text{ }}g\].
The given amount of nitrogen provided in the question is 7 grams. So, in order to understand the number of moles of nitrogen present in this sample, let us formulate a basic relation:
If, 28 g of nitrogen \[ \to \] 1 mole of nitrogen, then
      7 g of nitrogen \[ \to \] x mole of nitrogen
Hence, \[x = \left( 7 \right)\left( 1 \right)/\left( {28} \right)\]
     x = 0.25
hence, 0.25 moles of nitrogen is present in the given sample.

Hence, Option A is the correct option.

Note:
One mole is equal to the amount of substance that contains as many elementary units as there are atoms in 0.012 kg of carbon–12. The elementary units may be atoms, molecules, ions, radicals, electrons, etc., and must be specified.