Which gas has the smallest rate of diffusion?

• A. Hydrogen: $\text{H}_2$
• B. Ammonia: $\text{NH}_3$
• C. Carbon dioxide: $\text{CO}_2$
• D. Chlorine: $\text{Cl}_2$

• A. Molar mass of $\text{H}_2 = 2 \times 1.008 \text{ g/mol} = 2.016 \text{ g/mol}$
• B. Molar mass of $\text{NH}_3 = 14.01 \text{ g/mol} + (3 \times 1.008 \text{ g/mol}) = 17.034 \text{ g/mol}$
• C. Molar mass of $\text{CO}_2 = 12.01 \text{ g/mol} + (2 \times 16.00 \text{ g/mol}) = 44.01 \text{ g/mol}$
• D. Molar mass of $\text{Cl}_2 = 2 \times 35.45 \text{ g/mol} = 70.90 \text{ g/mol}$

The molar masses are:

• Hydrogen: $2.016 \text{ g/mol}$
• Ammonia: $17.034 \text{ g/mol}$
• Carbon dioxide: $44.01 \text{ g/mol}$
• Chlorine: $70.90 \text{ g/mol}$

Chlorine ($\text{Cl}_2$) has the largest molar mass ($70.90 \text{ g/mol}$).

According to Graham's Law, the gas with the largest molar mass will have the smallest rate of diffusion. Therefore, chlorine has the smallest rate of diffusion.

The final answer is $\boxed{\text{D}}$.