The degree of dissociation α = (D - d) / d(n-1), where D is the initial vapor density, d is the final vapor density, and n is the number of moles of products from one mole of reactant (for PCl₅ ⇌ PCl₃ + Cl₂, n=2). α = (104.16 - 62) / (62 * (2-1)) = 42.16 / 62 ≈ 0.68, or 68%.