Assuming that the required amount of chlorine is added, we may calculate the mass of chlorine present in the product by first calculating the amount of product that will be formed from 15 grams of phosphorus.
Phosphorus has an atomic mass of 31, so 15/31 moles of phosphorus are added.
Next, we see that due to the equimolar ratio, the moles of product formed are also 15/31
The molecular mass of phosphorus trichloride is 137 g/mol. Therefore, the mass of product is:
137 * 15/31
= 66.3 grams
Now we know that 15 grams of phosphorus are present, so the remaining amount must be chlorine.
Chlorine = 66.3 - 15
Chlorine = 51.3 grams
