The molecular formula of ammonia is NH3. Gram molecular mass is the sum of gram atomic mass of all the atoms present in the molecule.
∙ Gram atomic mass of nitrogen = 14 g
∙ Gram atomic mass of hydrogen = 1 g
Gram molecular mass of ammonia (NH3) = gram atomic mass of nitrogen + 3×gram atomic mass of hydrogen
Thus, gram molecular mass of ammonia = 14+(3×1) = 17 g