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

Answer:

Opposite of limiting reagent.

Explanation:

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.