Question

To an equilibrium mixture of $2S{O}_2\left(g\right)+O_2\left(g\right)\rightleftharpoons 2S{O}_3\left(g\right)$ some helium, an inert gas, is added at constant volume. The addition of helium causes the total pressure to double. Which of the following is true?

• A. The concentration of sulphur trioxide increases
• B. The number of moles of sulphur trioxide increases
• C. The concentration of sulphur dioxide increases
• D. The concentration of the three gases is unchanged

Answer 1

The correct option is D The concentration of the three gases is unchanged

Effect of addition of an inert gas at constant volume

On addition of Inert gas, as volume remains constant, the partial pressures of the three gases will remain unchanged upon addition of helium gas. Hence there will not be any change in the reaction quotient of the mixture and equilibrium stays unaffected.

If the volume remains constant and an inert gas such as $He,Ar,etc$. is added which does not take part in the reaction, the equilibrium remains undisturbed and consequently the concentrations of the products and reactants will not change.

Hence, option (C) is the correct answer.