Percent by Mass


Calculate the percent by mass of phosphorus and chlorine in phosphorus trichloride (PCl3).


  1. Assume one mole of phosporus trichloride. Find the moles of phosphorus and moles of chlorine.

    (1 mol PCl3) (1 mol P/ 1 mol PCl3) = 1 mol P

    (1 mol PCl3) (3 mol Cl/ 1 mol PCl3) = 3 mol Cl

  2. Convert mole of each element to mass of each element.

    (1 mol P) (30.97 g P/ 1 mol P) = 30.97 g P

    (3 mol Cl) (35.45 g Cl/ 1 mol Cl) = 106.4 g Cl

  3. Calculate the percent by mass of each element.


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