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Answer: 72.07%
Explanation:
P4 + 6 Cl2 --> 4PCl3
Since 123.90g/mol P4 is equivalent to 4(137.33g/mol PCl3)
So, 79.48g P4 = 4(137.33g/mol)(79.48g)/(123.90g/mol)
= 352.38g PCl3 formed
Percentage yield = (253.9/352.38) * 100 = 72.07%