How many moles of water will be produced
when 8 grams of hydrogen gas react with the
oxygen in the air?
Episode 801
Stoichiometry
• Study of the quantity (mass) relationships in a
chemical reaction
• Based on balanced equations
2Mg + O2  2MgO
• Two atoms of magnesium reaction with 1 oxygen
molecule to form 2 magnesium oxide formula
units
or
• Two moles of magnesium atoms react with 1
mole of oxygen molecules to form 2 moles of
magnesium oxide formula units
Episode 801
• The coefficients in a balance equation give the
MOLE RATIO for the substances involved in
the reaction.
– Example problem:
– When elemental aluminum reacts with elemental
iodine, aluminum iodide is produced.
2 Al + 3 I2  2 AlI3
• Mole ratios:
____Al:
2
2
____Al:
3 2:
____I
____I
3
2
2
____AlI
3
2
____AlI
3
• If you start with 4 moles of Al, how many moles of
AlI3 will be produced?
? mol AlI3 = 4 mol Al 2 mol AlI3
2 mol Al
= 2 mol AlI3
Episode 801
Problem Set 1
2 N2H4 +
N2O4  3 N2 + 4 H2O
BE SURE TO BALANCE THE EQUATION FIRST!!
? mol N2H4 = 2.72 mol N2H4
2.72 mol N2H4 1 mol N2O4
2 mol N2H4
= 1.36 mol N2O4
? mol N2 = 2.72 mol N2H4
2.72 mol N2H4
3 mol N2
2 mol N2H4
= 4.08 mol N2
Episode 801
How many moles of water will be produced
when 8 grams of hydrogen gas react with the
oxygen in the air?
2 H2 + O2  2 H2O
(Hint: We will need to change grams of H2 to moles H2)
? mol H2O = 8 g H2
1 mol H2 2 mol H2O
2 g H2
2 mol H2 =
4 mol H2O
(Hint: To “make the switch” between different substances in a
reaction, use the mole ratio from the balanced equation.)
Episode 801
Problem Set 2
In photosynthesis, carbon dioxide and water react to
form glucose, C6H12O6 and oxygen gas.
6 CO2 + 6 H2O  C6H12O6 + 6 O2
If 15.6g of carbon dioxide react, how many moles of glucose
will be produced?
? mol C6H12O6 = 15.6g CO2 1 mol CO2
44g CO2
1 mol C6H12O6
6 mol CO2
= 0.059 mol C6H12O6
How many grams of carbon dioxide must react to form 0.25
moles of glucose?
? g CO2 = 0.25 mol C6H12O6
6 mol CO2
44g CO2
1 mol C6H12O6
1 mol CO2
= 66g CO2
Episode 801
Challenge Problem
• When nitrogen and hydrogen react, they
form ammonia gas, which has the
formula NH3. If 56.0 g of nitrogen are
used up in the reaction, how many grams
of ammonia will be produced?
Episode 801