The addition of a small amount of argon at a constant volume will not affect the equilibrium of the following reaction:

1. H2(g)+I2(g)⇌2HI(g)
2. PCl5(g)⇌ PCl3(g)+Cl2(g)
3. N2(g)+3H2(g)⇌2NH3(g)
4. The equilibrium will remain unaffected in all the three cases.

Hint: Inert gas doesn't affect equilibrium at constant volume according to Le - Chatelier's principle.

Explanation:
Step 1:
Le- Chatelier's principle, states that changes in the temperature, pressure, volume, or concentration of a system will result in predictable and opposing changes in the system in order to achieve a new equilibrium state.
The reaction is as follows:
H2(g)+I2(g)2HI(g)
PCl5(g)PCl3(g)+Cl2(g)
N2(g)+3H2(g)2NH3(g)
Step 2:
According to Le chatelier's principle, the equilibrium constant (K) remains unaffected by the addition of inert gas in constant volume. Hence, in all three cases equilibrium remain unaffected.