Which of the following will diffuse slowest in air?

The correct option is Chlorine.

To understand why chlorine diffuses the slowest, we can use Graham's Law of Diffusion. Graham's Law states that the rate of diffusion of a gas is inversely proportional to the square root of its molar mass (or density) at constant temperature and pressure. Mathematically, this is expressed as:

$$\mathrm{Rate}\propto \frac{1}{\sqrt{M}}$$

where $M$ represents the molar mass of the gas. According to this relationship, a gas with a larger molar mass (heavier molecules) will diffuse slower than a gas with a smaller molar mass (lighter molecules).

Let's find the molar masses of all the given gases to compare them:

1. Hydrogen (H₂): The molar mass is approximately 2 g/mol.
2. Ammonia (NH₃): The molar mass is approximately 14 + (3 × 1) = 17 g/mol.
3. Oxygen (O₂): The molar mass is approximately 2 × 16 = 32 g/mol.
4. Chlorine (Cl₂): The molar mass is approximately 2 × 35.5 = 71 g/mol.

Comparing these molar masses, we see that:

$$M_{H_2}<M_{N{H}_3}<M_{O_2}<M_{C{l}_2}$$

Since chlorine (Cl₂) has the highest molar mass (71 g/mol) among the choices, it consists of the heaviest molecules and will therefore diffuse the slowest in air.