# Moles, excess and limiting reagents

### Moles, excess and limiting reagents

#### Lessons

In this lesson, we will learn:
• To fully understand the language used to describe chemical reactions.
• To identify by calculation the limiting reagents in a given chemical reaction.
• To calculate quantities of excess reagents.

Notes:

• For a chemical reaction to happen, all the reactants must be present and available to react. If any one reactant is not present, the reaction will not happen. This means in practice that chemical reactions happen until one of the reactants runs out. When this happens, the reaction stops and no more products can be made.

• The chemical that you have the least amount of, or that runs out first is called the limiting reagent because its running out limits the reaction from happening any longer. All the other reagents (reactants) involved that are not limiting reagents are excess reagents or are “in excess”. We call it this because when the limiting reagent runs out, there will still be some of this reagent left over – there was an excessive amount of it.

• To find out the limiting reagent, you need to find the amount of product that can be made, with respect to each reactant involved. The reactant that would produce the smallest amount of product is the limiting reagent.

• To find the mass of excess reagent, find the amount of the excess reagent that reacts based on the amount of limiting reagent. Then, subtract that from the total amount of excess reagent available.

• Knowing your limiting reagent is important because a limit on the amount of reagents available puts a limit on the amount of products you can make too!
• Introduction
a)
Recap on mole/mass/volume/concentration equations

b)
Assumptions we made in previous videos

c)
Definition of terms in chemical reactions.

d)
Why one reagent is the most important.

• 1.
Find the limiting reagent in a chemical reaction with known quantities.
Study the reaction: 2H$_2 +$O$_2$→2H$_2$O
a)
50 g of O$_2$ gas and 50 g of H$_2$ gas were reacted together.
Which reagent is the limiting reagent?

b)
What mass of the other reagent is in excess?

• 2.
Find the limiting reagent in a chemical reaction with known quantities.
Study the reaction: 2C$_6$H$_{14} +$19O$_2$→12CO$_2 +$14H$_2$O
a)
If 120 g of O$_2$ and 150 g of C$_6$H$_{14}$ are reacted together, what is the limiting reagent?

b)
How many grams of the excess reagent are present in excess?

• 3.
Find the limiting reagent in a chemical reaction with known quantities.
500 g of Fe$_2$O$_3$ is reacted with 750 g of C in the reaction: 2Fe$_2$O$_{3\;(s)} +$3C$_{\;(s)}$→4Fe$_{\;(s)} +$3CO$_{2\;(g)}$
a)
What mass of Fe is produced?

b)
What is the limiting reagent in this reaction?

c)
How many extra grams of this reagent are in excess?

• 4.
Find the limiting reagent in a chemical reaction with known quantities.
45 g of Ca$_3$(PO$_4$)$_2$ is reacted with 36 g C and 85 g SiO$_2$ according to the reaction:
2Ca$_3$(PO$_4$)$_2 +$6SiO$_2 +$10C→P$_4 +$6CaSiO$_3 +$10CO
a)
What mass of P$_4$ can be made from these quantities?

b)
What is the limiting reagent?

c)
Find the excess mass of both excess reagents.