Ch. 11: Stoichiometry
Sec. 11.2: Stoichiometric
Calculations
Objectives
• List the sequence of steps used in solving
stoichiometric problems.
• Solve stoichiometric problems.
Using Stoichiometry
• Stoichiometry is the study of quantitative
relationships between the amounts of
reactants used and the amounts of products
formed by a chemical reaction.
• In Action: Start with a balanced equation;
develop mole ratios between “given” (or
“known”) and “unknown”; use molar
conversions to convert to appropriate unit.
Mole to Mole Conversions
Problem: Potassium reacts with water to
produce potassium hydroxide and hydrogen
gas. Determine the number of moles of
hydrogen gas produced when 0.04 mol of K
is used?
Mole to Mole Conversions
• Step 1: Write a balanced equation.
2K(s) + 2H2O(l)
2KOH(aq) + H2(g)
• Step 2: Identify the known and unknown in the
problem.
– 0.04 mol K is known; mol H2 is unknown.
• Step 3: Use the balanced equation to determine the
mole ratio in which mol unknown is the numerator
and mol known is the denominator.
mol unknown = 1 mol H2
mol known
2 mol K
Mole to Mole Conversions
• Step 4: Multiply the known by the mole
ratio. Note that known substance & its units
will cancel if it is correctly done.
0.04 mol K x 1 mol H2 = 0.02 mol H2
2 mol K
We have gone from moles of known
to moles of unknown!
Practice Problems
• How many moles of carbon dioxide gas are
produced when 10.0 mol of propane (C3H8)
are burned in an excess of oxygen?
• Sulfuric acid is produced when sulfur
dioxide reacts with oxygen gas and water.
If 12.5 mol SO2 reacts, many many mol of
H2SO4 are produced? How many mol of O2
are needed?
Mole to Mass Conversions
• Follow steps 1 - 4 (for mole to mole
conversions).
• Add an extra step: Use a molar conversion
to convert from moles to mass or from mass
to moles where needed in the problem.
Practice Problems
• Determine the mass of NaCl produced when
1.25 mol Cl2 reacts with Na.
• How many grams of chlorine gas can be
obtained from the decomposition of 2.50
mol of NaCl?
* Determine the number of moles of NaCl
required to obtain 3.4 g of sodium when
NaCl is decomposed.
Mass to Mass Conversions
• What do we need to do in a mass to mass
conversion?
– Convert a given mass to moles.
– From moles known, calculate moles unknown.
– Convert moles unknown to mass unknown.
29
29
29
29
8.6g
In Summary
1 - Convert mass of “known” to moles.
2 - Do a mole to mole conversion between
moles of known & moles of unknown.
• Write a balanced equation.
• Identify the known and unknown.
• Use the balanced equation to determine the mole
ratio in which mol unknown is the numerator and
mol known is the denominator.
• Multiply the known by the mole ratio.
3 - Convert moles of unknown to mass.
Practice Problems
• Ammonium nitrate, an important fertilizer,
produces dinitrogen oxide and water when
it decomposes. Determine the mass of
water produced from the decomposition of
25.0 g of ammonium nitrate.
• The decomposition of NaN3 produces the
nitrogen gas that inflates air bags.
Determine the mass of N2 produced if
100. g NaN3 is decomposed.
Gas Stoichiometry (pgs. 440-1)
• Objectives
– Determine volume ratios for gaseous
reactants and products by using
coefficients from a chemical equation.
– Calculate amounts of gaseous reactants
and products in a chemical reaction
Volume to Volume Conversions
• Recall that coefficients in chemical
reactions can be interpreted as particles,
moles, & mass.
• Since Avogadro’s principle states that equal
volumes of gases at the same temperature &
pressure contain equal numbers of particles,
the coefficients can also represent relative
volumes when gases are involved.
Volume to Volume Conversions
• 2C4H10(g) + 13O2(g) 8CO2(g) + 10H2O(g)
• Interpreted in terms of volumes:
– 2 L butane reacts with 13 L of oxygen gas to
produce 8 L of carbon dioxide and 10 L of
water vapor.
OR
– 2 mL butane reacts with 13 mL of oxygen gas
to produce 8 mL of carbon dioxide and 10 mL
of water vapor.
Volume to Volume Conversions
• In order to find the volume of a reactant or
product in a reaction, you will need a
balanced chemical equation and the volume
of at least one gas involved in the reaction.
• Because the coefficients represent volumes,
we can form volume ratios (much like mole
ratios) from the balanced equation.
• We will then follow essentially the same
steps as in mole-mole stoichiometric
calculations.
CH4(g) + 2O2(g) CO2(g) + 2H2O(g)
• What volume of methane is needed to
produce 26L of water vapor?
1 -Equation is balanced.
2 -Known: 26L H2O; Unknown: Vol of methane.
3 -Volume ratio between unknown and known is:
1 L CH4
2 L H2O
4 -Multiply known by ratio: 26 L H2O x 1 L CH4 =
2 L H2O
13 L CH4
Practice Problems
• What volume of oxygen is needed to react
with solid sulfur to form 3.5 L of sulfur
dioxide?
• Determine the volume of hydrogen gas
needed to react completely with 5.00 L of
oxygen to form water.
• How many liters of propane gas (C3H8) will
undergo complete combustion with 34.0 L
of oxygen gas?
Advanced Gas Stoichiometry
• If conditions are at STP, it is possible, using
gas stoichiometry and mole conversions, to
calculate moles, mass, and/or #’s of
particles of gas from a given volume of gas
in a chemical reaction.
• (We can also calculate volume from moles,
mass, and/or #’s of particles.)
NH4NO3(s) N2O(g) + 2H2O(g)
• Calculate the mass of ammonium nitrate
that must be used to obtain 0.100 L of
dinitrogen gas at STP.
– Equation is balanced.
– Known: 0.100 L N2O;
Unknown: mass of NH4NO3
* Since it is not possible to get a volume ratio
with a solid, we will need to use a mole ratio
between the unknown and known (1 mol
NH4NO3/1 mol N2O) AFTER we convert 0.100
L to moles!
NH4NO3(s) N2O(g) + 2H2O(g)
• Convert L to moles
0.100 L N2O x 1 mole = 0.00446 mol N2O
22.4 L
• Use the mole ratio between known & unknown
0.00446 mol N2O x 1 mol NH4NO3 =
1 mol N2O
0.00446 mol NH4NO3
• Convert from moles to grams
0.00446 mol NH4NO3 x 80 g = 0.357 g NH4NO3
1 mol
Practice Problems
• When calcium carbonate is heated, it
decomposes to form solid calcium oxide and
carbon dioxide gas. How many liters of gas are
produced at STP if 2.38 kg of calcium
carbonate reacts completely?
• Iron rusts by reacting with oxygen to form iron
(III) oxide. Calculate the volume of oxygen gas
at STP that is required to completely react with
52.0 g of iron.