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Stoichiometry
Chapter 11
Stoichiometry = the study of quantitative
relationships between the amounts of
reactants used and products formed by a
chemical reaction
– Based on the Law of Conservation of Mass
• Mass is neither created nor destroyed in a
chemical reaction
Law of Conservation of Mass
4Fe + 3O2  2Fe2O3
Reactants
4 mol Fe x 55.85g Fe = 223.4g Fe
1 mol Fe
3 mol O2 x 32.00g O2 = 96.00g O2
1 mol O2
Total 319.4 g
Products
2 mol Fe2O3 x 159.7g Fe2O3 = 319.4 g Fe2O3
1 mol Fe2O3
Total 319.4 g
Balancing Equations Review
•
Change the coefficients to make the
number of atoms of each element equal
on both sides of the equation
*NEVER change a subscript…doing so
changes the identity of the substance
•
•
Write the coefficients in their lowest
possible ratio
Check your work
A few things you do NOT do…
H2 + O2 ---> H2O
1. You cannot change a subscript
H2 + O2 ---> H2O2
2. You cannot place a coefficient in the
middle of a formula
H2 + O2 ---> H22O
3. Make sure that your final set of
coefficients are all whole numbers with
no common factors other than one
4 H2 + 2 O2 ---> 4 H2O
Balancing Equations Review
Balance this equation
NaOH(aq) + CaBr2(aq)  Ca(OH)2(s) + NaBr(aq)
2NaOH(aq) + CaBr2(aq)  Ca(OH)2(s) + 2NaBr(aq)
Review: Mole Ratio or
(Conversion Factors)
Mole Ratio = ratio between the numbers of
moles of ANY two substances in a
balanced chemical equation
2Al + 3Br2 2AlBr3
What mole ratios can be written for this reaction?
2mol Al
3mol Br2
3molBr2
2mol AlBr3
2molAl
2molAlBr3
3mol Br2
2mol Al
2molAlBr3
2molAl
2molAlBr3
3molBr2
Helpful Hints
• ALWAYS make sure the chemical
equation is balanced!!!
• Moles are ALWAYS involved when solving
stoichiometric problems
• The mole ratio is needed to convert from
one substance to another in a balanced
chemical equation
• If you are in doubt about how to proceed in
solving a problem,
GO TO MOLES FIRST!!
Mole to Mole Conversions
How many moles of hydrogen are produced
when 0.0400 mole of potassium is used?
2K + 2H20  2KOH + H2
1. Identify the known… K
2. Identify the unknown…H2
3. To solve this problem, you need to know how
the unknown moles of H2 are related to the
know moles of K
4. The correct ratio should have the moles of
unknown in the numerator and the moles of
the known should be in the denominator
1mol H2
2mol K
5. This mole ratio can be used to convert the
known number of moles of K to a number of
moles of H2
0.0400mol K x 1mol H2 = 0.0200mol H2
2mol K
If you put 0.0400mol K into water,
0.0200mol H2 will be produced
Try one…
C3H8 + 5O2  3CO2 + 4H2O
How many moles of CO2 are produced when
10.0 moles of propane are burned in
excess oxygen in a gas grill?
Mole to Mass Conversions
• A mole to mass conversion is done when
you know the number of moles of a
reactant/product and you want to calculate
the mass of another reactant/product
Example:
Determine the mass of NaCl produced when
1.25 moles of chlorine gas reacts
vigorously with sodium.
2Na + Cl2  2NaCl
Unknown…NaCl
2. Known…Cl2
3. Mole ratio…. 2mol NaCl
1 mol Cl2
4. Multiply the given by the mole ratio
1.25mol Cl2 x 2mol NaCl = 2.50mol NaCl
1 mol Cl2
1.
Example: continued…
Determine the mass of NaCl produced
when 1.25 moles of chlorine gas
reacts vigorously with sodium.
2Na + Cl2  2NaCl
5.
Multiply the moles of NaCl by the molar mass of NaCl
2.50mol NaCl x 58.45g NaCl =
1mol NaCl
= 146g NaCl produced
Try one…
Sodium chloride is decomposed into the
elements sodium and chlorine by means
of electrical energy. How many grams of
chlorine gas can be obtained from 2.50mol
NaCl?
2NaCl 2Na + Cl2
Mass to Mass Conversions
Example:
Determine the mass of H2O produced from
the decomposition of 25.0g of solid
ammonium nitrate.
NH4NO3  N2O + 2H2O
3.
1. Unknown…H2O
2. Known… NH4NO3
Convert the grams of the given to moles
25.0g NH4NO3 x 1mol NH4NO3
80.06g NH4NO3
= 0.312mol NH4NO3
Example:
Determine the mass of H2O produced from
the decomposition of 25.0g of solid
ammonium nitrate.
NH4NO3  N2O + 2H2O
4. Mole ratio… 2mol H2O
1mol NH4NO3
5. Multiply by the moles of NH4NO3 previously calculated
0.312mol NH4NO3 x 2mol H2O = 0.624mol H2O
1mol NH4NO3
Example:
Determine the mass of H2O produced from
the decomposition of 25.0g of solid
ammonium nitrate.
NH4NO3  N2O + 2H2O
6.
Calculate the mass of H20 using the molar mass
conversion factor
0.624mol H2O x 18.02g H2O = 11.2g H2O
1mol H2O
Courtesy of
Chemistry Matter and Change
Glencoe McGraw-Hill
Chemistry for Dummies
John T. Moore
www.karentimberlake.com