To find the number of neutrons in a atom, you subtract the number of_____ from the mass number.

You can simply subtract the atomic number from the mass number in order to find the number of neutrons.

  39.4 g

Explanation:

Of the 137.33 grams per mole of PCl₃, 106.36 grams of it are chlorine, about 77.4%. Thus the mass of chlorine in the 50.9 grams of PCl₃ will be ...

   (50.9 g)(0.7745) ≈ 39.4 g

39.4 g of chlorine reacted with the available phosphorus to produce 50.9 g of phosphorus trichloride.

__

P = 30.974 g/mol

Cl = 35.453 g/mol

Comment on answer choices

Since our answer doesn't match any of the offered choices, we assume there is a typo in the question. The best choice of those shown here is 35.4 g. A little more than 3/4 of the product weight is chlorine, so the answers 66.4 and 17.7 or lower do not make any sense.