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

To determine the number of molecules, we calculate the number of moles (n = mass / molar mass). For diatomic gases at normal temperature and pressure: 3g of Hydrogen (H2, molar mass 2g/mol) equals 1.5 moles. 48g of Oxygen (O2, molar mass 32g/mol) equals 1.5 moles. 42g of Nitrogen (N2, molar mass 28g/mol) equals 1.5 moles. In contrast, 2g of Carbon (C, atomic mass 12g/mol) equals approximately 0.167 moles. Since the number of molecules (or atoms for monoatomic substances) is directly proportional to the number of moles via Avogadro's number (6.022 x 10^23), the first three options all contain 1.5 times Avogadro's number of molecules. Option 4 has a significantly lower number of particles, making it the distinct choice.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 4: Carbon and its Compounds > 4.2.4 Homologous Series > p. 66