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:

The correct option is (D), (C), (A), (B).

To determine the decreasing order of the number of molecules, we use the relation between mass, molar mass, and the number of moles. Since the number of molecules is directly proportional to the number of moles, we can find the number of moles for each substance using the formula:

$\text{Number of moles}=\frac{\text{Given mass (g)}}{\text{Molar mass (g/mol)}}$
Here, the given mass is 16 g for all the substances.

Step-by-step calculations:

(A) 16 g of O₂:
Molar mass of O₂ = 2 × 16 = 32 g/mol

$\text{Moles of}{\text{O}}_2=\frac{16}{32}=0.5\text{mol}$

(B) 16 g of CO₂:
Molar mass of CO₂ = 12 + (2 × 16) = 44 g/mol

$\text{Moles of}{\text{CO}}_2=\frac{16}{44}\approx 0.36\text{mol}$

(C) 16 g of CO:
Molar mass of CO = 12 + 16 = 28 g/mol

$\text{Moles of CO}=\frac{16}{28}\approx 0.57\text{mol}$

(D) 16 g of H₂:
Molar mass of H₂ = 2 × 1 = 2 g/mol

$\text{Moles of}{\text{H}}_2=\frac{16}{2}=8\text{mol}$

Comparing the values, the order of the number of moles (and hence molecules) from highest to lowest is:
H₂ (8 mol) > CO (0.57 mol) > O₂ (0.5 mol) > CO₂ (0.36 mol)

Therefore, the decreasing order is (D), (C), (A), (B).