Arrange the following in decreasing order of number of molecules contained in :
(A) 16 g of O2
(B) 16 g of CO2
(C) 16 g of CO
(D) 16 g of H2
Choose the correct order from the options given below :
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (D), (C), (A), (B)
The number of molecules is proportional to the number of moles (N = n × NA). For the same mass, the number of moles is mass divided by molar mass, so the substance with the smallest molar mass has the most molecules.
\( \displaystyle n=\frac{m}{M} \)
(A) 16 g of \( \ce{O2} \) (M = 32 g mol−1): 16/32 = 0.50 mol — third
(B) 16 g of \( \ce{CO2} \) (M = 44 g mol−1): 16/44 = 0.36 mol — last
(C) 16 g of CO (M = 28 g mol−1): 16/28 = 0.57 mol — second
(D) 16 g of \( \ce{H2} \) (M = 2 g mol−1): 16/2 = 8 mol — first
Shortcut: For equal masses, the order of molecules is the reverse of the order of molar masses: \( \ce{H2} \) (2) < CO (28) < \( \ce{O2} \) (32) < \( \ce{CO2} \) (44).