Why do some chemistry questions say "assume there is excess [x]"?

For instance, there was a chemistry problem I had to do that asked me to find the mass of a compound through a balanced equation. At the end of the question, it says "Assume that there is excess Pb(NO_3)_2". I just disregarded it, and was able to solve the problem anyways, but I want to know why it was there in the first place.

1 Answer
Dec 17, 2017


Opposite of limiting reagent.


By assuming one of the reagents is in excess, we will then know that the other reagent is the limiting reagent.
For example, in this equation:
A + B —> C + D
Assume A is in excess, then B will be the limiting reagent. It means that no matter how much A you add, the reaction will stop producing C and D once you finish using up B. This means that the amount of B determines the yield of products.
Then from there, you calculate the number of moles of B and by using mole ratio, compare it with C and D, and you will be able to find either the volume or mass of the products.