Which one of the following has different number of molecules? (All kept at normal temperature and pressure)

The correct answer is option 4 i.e. 2 grams of carbon.

Number of moles

• The number of moles can be calculated by Mass (g) divided by Molecular mass.
• \(Number\space of\space moles = {Mass(g) \over Molecular\space mass\space (M)}\)
• We know the same number of moles have the same number of molecules or atoms.
• 3 grams of Hydrogen = 1.5 moles, because the molecular mass of Hydrogen is 2.
• 48 grams of Oxygen= 1.5 moles, because the molecular  mass of Oxygen is 32.
• 42 grams of Nitrogen = 1.5 moles, because the molecular mass of Nitrogen is 28.
• 2 grams of carbon = 0.167 moles, because the molecular mass of carbon is 14.
• Hence option 4 is correct.