How do I distinguish between the limiting reactant and excess reactant in a chemical equation?

Answer 1

YOu have to look at the balanced reaction, then see how many moles of each reactant you have. For instance

3A+ B >>>

and you have 4 moles of A , and 1.4 moles of B.

The balanced reaction requires if you have 1.4 moles of b, you should have 3*1.2=4.2 moles of A, and you don't have that. So A is limiting, you will run out of it. THe excess is B: how much excess? 4/3=1.33 moles of B needed, so excess is 1.4-1.33 moles.