So,
What we're going to do to solve this problem is to identify the limiting reagent first. To do this, we're first given that 12.05 g of phosphorus reacted with 61.68 g of bromine to form phosphorus tribromide.
The first thing we need to do to find the limiting reagent, is to pass all amounts in grams to moles. (We do this dividing these amounts by the molecular weight of the compound:
Now that we have passed all our amounts into moles, what we do is to use the ratio of the coefficients of the reaction.
The reaction that takes place in this problem is the next one:
What we're going to do now is to use the ratio of the coefficients of P and Br2 with the amounts in moles that we have. According to the reaction, we can see that per each 2 moles of P, we need 3 moles of Br2 to react.
Using this:
If we analyze, we conclude that:
- We have 0.772 moles of Br2, but according to the reaction, we would only need 0.583 moles of Br2.
- We have 0.389 moles of P, but according to the reaction we would need 0.514 moles of P.
You can notice that we would need more than the amount of P that we have. So, P is the limiting reagent.
Now that we know that P is the limiting reagent, we use this fact to find the amount of PBr3 formed:
Therefore, 105.3 grams of PBr3 would be formed if 12.05 g of phosphorus reacted with 61.68 g of bromine.