Question

If PCl$_5$ is heated in two separate vessels of volume 5 L and 10 L respectively at 27$^\circ$C, the extent of dissociation of PCl$_5$ will be

• A. More in 5 L vessel
• B. More in 10 L vessel
• C. More in 8 L vessel
• D. Cannot be said

Hint:

For gaseous equilibria, increasing volume (decreasing pressure) favors the side with \textbf{more moles of gas}.

Answer 1

The Correct Option is B

Step 1: Write the dissociation reaction of phosphorus pentachloride: \[ \text{PCl}_5 (g) \rightleftharpoons \text{PCl}_3 (g) + \text{Cl}_2 (g) \]
Step 2: Count the number of gaseous molecules on each side: \[ \text{Left side} = 1 \text{ mole}, \quad \text{Right side} = 2 \text{ moles} \]
Step 3: According to Le Chatelier’s principle, increasing volume (or decreasing pressure) favors the side with more number of gaseous molecules.
Step 4: Since dissociation produces more gas molecules, a larger volume will favor greater dissociation.
Step 5: Therefore, the extent of dissociation of PCl$_5$ will be more in the 10 L vessel.