Question

calculate the mass of 0.5 mole of nitrogen gas

Answer 1

nitrogen gas is N2.
Molecular mass of N2 is 28 g
1 mole weighs 28g
0.5 moles weigh 14g

Answer 2

Nitrogen molecule has 2 atoms.
Atomic number of Nitrogen is 7.  Hence, Its molecular mass = 14 amu (7 protons and 7 neutrons.

1 mole of N2 gas contains Avogadro number of molecules: N = 6.022 * 10^23
mass of one molecule = 28 amu

mass of one mole of molecules of N2 = 28  gm/mol
mass of 0.5 mole of molecules of N2  at STP =  0.5 * 28 = 14 gm