The reaction of phosphorus with chlorine to make phosphorus trichloride has a yield of 78%. in order to produce 512 g of phosphorus trichloride how many grams of phosphorus will you need?

Question

P4 + 6Cl2 —> 4PCl3

1. 512.00g
2. 147.90g
3. 656.00g
4. 591.60g
5. 137.50g

Answer 1

Hello, Caitlyn,

The balanced equation tells us that, under ideal conditions (theoretical) we'll get 4 moles of PCl₃ for every 1 mole of P₄. The molar ratio is 4 moles PCl₃/mole P₄. Or, 0.25 moles P₄/mole PCl₃.

We want to wind up with 512g of PCl₃, but we're told this reaction only has a 78% yield. Phooey. (the technical term for this level of yield). So let's plan ahead and adjust the amount of ending material we should target for by dividing the 512 grams by 0.78, which tells us we should target for 656.4 grams of PCl₃, so that we wind up with (0.78*656.4) 512 grams.

656.4 grams of PCl₃ is 4.78 moles of the material (656.4g/137.3g/mole). Multiply this by the molar ratio we deduced earlier (0.25 moles P₄/PCl₃) to find the moles P₄ we need.

(0.25 moles P₄/PCl₃)*(4.78 moles PCl₃) = 1.195 moles P₄. Convert to grams by multiplying by the molar mass of P₄. That gives us 148.1 grams P₄. Slightly different than option 2, but that may be due to a difference in atomic masses or rounding.

Bob