The effect of addition of helium gas to the following reaction in equilibrium state at constant volume, is:
$$\text{PCl}_5(g) \rightleftharpoons \text{PCl}_3(g) + \text{Cl}_2(g)$$

The equilibrium reaction is: $$PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g)$$

When helium gas is added at constant volume:

1. The total pressure increases, but the partial pressures of the reacting gases remain unchanged.

2. Since the equilibrium constant $$K_p$$ depends on the partial pressures of the reacting gases (not the total pressure), and the partial pressures haven't changed, the system remains at equilibrium.

3. Similarly, at constant volume, the concentrations of the reacting species are unchanged, so $$K_c$$ is also unaffected.

4. Helium is an inert gas and does not participate in the reaction.

Therefore, the addition of helium at constant volume will not affect the equilibrium.

Note: If helium were added at constant pressure (instead of constant volume), the volume would increase, effectively decreasing the concentrations, which would shift the equilibrium forward.