12.3g of P4 reacts with cl2 to produce 50.0g of PCl3. what is the percent yield?

Molar mass of P₄ = 31.0×4 g/mol = 124.0 g/mol
Molar mass of PCl₃ = (31.0 + 35.5×3) g/mol = 137.5 g/mol

Balanced equation for the reaction:
P₄ + 12Cl₂ → 4PCl₃
Mole ratio P₄ : PCl₃ = 1 : 4

Moles of P₄ reacted = (12.3 g) × (1 mol / 124.0 g) = 0.09919 mol
Maximum moles of PCl₃ produced = (0.09919 mol) × 4 = 0.3968 mol
Theoretical yield of PCl₃ = (0.3968 mol) × (137.5 g/mol) = 54.6 g

Percent yield = (50.0/54.6) × 100% = 91.6%

====
OR:

(12.3 g P₄) × (1 mol P₄ / 124.0 g P₄) × (Max. 4 mol PCl₃ / 1 mol P₄) × (137.5 g PCl₃ / 1 mol PCl₃)
= Max. 54.6 g PCl₃

(50.0/54.6) × 100%
= 91.6% yield