Percent by Mass

Problem:

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

Strategy:

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.
   
   Mass of sample = 30.97 g + 106.4 g = 137.4 g

   Percent P = [(30.97 g) / (137.4 g)] (100%) = 22.54%

   Percent Cl = [(106.4 g) / (137.4 g)] (100%) = 77.44%