Which of the following contains the maximum number of molecules?

(a) 4 g of O₂

(b) 4 g of NH₃

(c) 4 g of CO₂

(d) 4 g of SO₂

4 g of NH₃

Reason —

(a) No. of molecules in 4 g of O₂

Molecular wt. of any substance contain 6.022 × 10²³ molecules.

Mass of 1 mole of O₂ is 2(16) = 32 g

32 g of O₂ contains 6.022 × 10²³ molecules

∴ 4 g of O₂ contains $\dfrac{6.022 \times 10^{23} \times 4}{32}$

= 7.5 x 10²² molecules.

Similarly,

(b) 4 g of NH₃ [14 + 3 = 17g ] contains $\dfrac{6.022 \times 10^{23} \times 4}{17}$

(c) 4 g of CO [12 + 16 = 28g ] contains $\dfrac{6.022 \times 10^{23} \times 4}{28}$

(d) 4 g of SO₂ [32 + 32 = 64g ] contains $\dfrac{6.022 \times 10^{23} \times 4}{64}$

∴ 4g of NH₃ having minimum molecular mass contains maximum molecules.

Note : The fraction with lowest denominator gives the highest value. Hence, by observation we can say that 4 g of NH₃ has maximum number of molecules.