Question: how many chlorine atoms are there in four phosphorus trichloride molecules?
Answer: Let's balance the chlorine now. On the left-hand side I only have two chlorine atoms. This one molecule of chlorine has two atoms in it. Here I have, each molecule of phosphorus trichloride has three chlorines, and I have four molecules of it, so four times three, I have 12 chlorines on the right-hand side.