Question

Determine the theoretical yield of C when each of the following amounts of A and B are allowed to react in the generic reaction: A+2B-->3C

1) 1mol A; 1 mole B
2) 1mole A; 3 mole B
3) 2mole A; 2 mole B
4) 32mole A; 68 mole B

Answer 1

I will do 1) for you in detail.
A + 2B ==> 3C

mols A = 1. How many moles of C will that form? Using the coefficients in the balanced equation, convert moles A to moles C. 1 mole A x (3 moles C/1 mole A) = 3 moles C.

moles B = 1. How many moles of C will that form? Same process as above, convert moles B to moles C. 1 mole B x (3 moles C/2 moles B) = 3/2 = 1.5 moles C.
3 moles C and 1.5 moles C don't agree and one of those answers must be wrong. The correct answer in limiting reagent problems is ALWAYS the smaller one and the reagent producing that value is the limiting reagent. So 1.5 moles C is the theoretical yield and B is the limiting reagent. (The question doesn't ask but we can calculate how much A isused up and how much is un-reacted). That will be 1 mole B x (1 mole A/2 moles B) = 1/2 or 0.5 mole A used; therefore, there is 0.5 mole A remaining un-reacted.)