Limiting Reactants

2. You have 100 bolts, 150 nuts, and 150 washers. You assemble a nut/bolt/washer set using the
following recipe or equation.
2 washers + 1 bolt + 1 nut 1 set
a) How many sets can you assemble from your supply?
You can assemble 75 sets from the supply.
B:N:W have: 100:150:150
need: 1:1:2 or 75:75:150
b) Which is the limiting component?
The washers are the limiting component.
3. This reaction of hydrogen with oxygen to produce water is described by the following recipe or
reaction equation, which says that 2 molecules of hydrogen react with 1 molecule of oxygen to
produce 2 molecules of water.
2H 2 + O 2
2H 2 O
You react 150 H 2 molecules with 100 O 2 molecules to produce H 2 O. Which is the limiting
reactant, hydrogen or oxygen? How many water molecules can you produce from your supply
of hydrogen and oxygen?
The limiting reactant is the H 2 . You can produce 150 water molecules from this supply of
hydrogen and oxygen.
H 2 :O 2 Have 150:100
Need 2:1, or 150:75
Model 2: Limiting Reactant in Moles
The reaction equation can be interpreted in terms of the number of molecules (see Exercise 3 above)
or moles reacting (2 moles of hydrogen react with 1 mole of oxygen to produce 2 moles of water).
The only difference is that while a mole can be divided into fractions of a mole, a molecule cannot
be divided. Each square below represents one mole of hydrogen, oxygen, and water.
Figure 7.2
1 mole of H 2 O
1 mole of H 2 1 mole of O 2
1 mole of H 2 1 mole of O 2 ½ mo
When these react according to the reaction equation,
2H 2 + O 2 2H 2 O
44
Foundations of Chemistry