In a $100 \mathrm{~L}$ vessel, 3 moles of nitrogen and 3 moles of $\mathrm{PCl}_5$ are taken and heated to $500 \mathrm{~K}$. The equilibrium pressure is $3.28 \mathrm{~atm}$. The percentage degree of dissociation of $\mathrm{PCl}_5$ is : (Assume ideal behaviour for all gases).