Answer:
 mass of Cl₂ = 1.46 g
Step-by-step explanation:
given data
mass of phosphorus trichloride = 3.66 G
mas of chlorine gas = ?
efficiency = 83.7 %
balanced chemical equation
 2P + 3Cl₂ → 2PCl₃
The mole ration of chlorine and phosphorus trichloride = 3 : 2
First we will find out number of moles of Cl₂ and PCl₃ 
As we know that:
 number of moles = mass / molar mass
 number of moles of PCl₃ = 3.66 g / 137.33 g.mol-
 number of moles of PCl₃ = 0.027 mol
As the mole ratio of PCl₃ and Cl₂ = 2 : 3
 so number of moles of Cl₂ = (0.027 ×3) / 2
 number of moles of Cl₂ = 0.041
Now we find the mass of Cl₂
 mass of Cl₂ = moles × molar mass
 mass of Cl₂ = 0.041 mol × 35.5 g/mol
 mass of Cl₂ = 1.46 g