[mburt]

Hi again,

I'm having some difficulty with a question in my textbook regarding the actual, theoretical, and percent yield of a reaction.

P₄(s) + 6Cl₂(g) 4PCl₃(l)

The question: What is the percent yield if the reaction of 25.0 g P₄ and 91.5 g Cl₂ produces 104 g PCl₃?

My attempt:

Well obviously the actual yield of PCl₃ in this case is 104 g, but my issue is with calculating the theoretical yield. You could use the mole ratio to discover the theoretical yield per mole of PCl₄, but I'm not sure how that would help you in comparing with the actual yield.

So my next guess was to convert the following:

25.0 g P₄ mol P₄ mol PCl₃ g PCl₃

Which turned out to be 111 g PCl₃- so right now it looks alright to me, the theoretical being larger than the actual yield. My logic was using the amount of one of the reactants to find the amount of product- aka the theoretical method.

But then I tried converting the other reactant, 91.5 g Cl₂, to g PCl₃ using the same pathway:

91.5 g Cl₂ mol Cl₂ mol PCl₃ g PCl₃

But this resulted in 118 g PCl₃. Here is where I'm stumped.

Am I supposed to use a weighted average of the amount of product or something? Right now it seems I have two theoretical yields- 111 g and 118 g. All the exercises prior to this only dealt with yield per mole of product so I really don't know where to go from here.

Thanks again guys

[Hunter2]

This exercise is conziniung the case of the limited reactants.  Check with the given values what drives the reaction. The other calculation is then not valid.

[Borek]

25.0 g P₄ mol P₄ mol PCl₃ g PCl₃

Which turned out to be 111 g PCl₃

91.5 g Cl₂ mol Cl₂ mol PCl₃ g PCl₃

But this resulted in 118 g PCl₃. Here is where I'm stumped.

Both calculations are in a way correct - that is, each number would make sense assuming the other reactant is in excess. But you know one of them is limiting, so only one of these results can be the right theoretical yield in this particular question. Can it be the larger one? That would mean reaction yield was not limited - so it has to be the lower one.

Actually when I was at school (before dinosaurs) we were taught to calculate both possible outcomes and to select the lower one.

[mburt]

Thanks guys, that makes perfect sense. I just wasn't connecting the dots between the two excercises- the theoretical yield turns out to be pretty logical.

I am slowly realizing that I enjoy chemistry! Ha ha!