STOICHIOMETRY
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* Stoichiometry - the study of quantitative relationships in chemical formulas and equations
* Chemical Formulas - a series of symbols and numbers used to represent the composition of an
element or a compound
I. Formula Mass - a measure of the mass in one mole of molecules (or formula units) of that
substance
16.0 amu
O
H
1.0 amu
H
1.0 amu
1.0 amu
1.0 amu
* units for these calculations are atomic mass units (amu) = grams
per mole (g/mol)
* The atomic mass of carbon is ~12.01 g/mol. This means that one
mole (6.02 x 1023) atoms of carbon will weigh about 12.01 g.
* The formula mass of carbon dioxide (CO2) is ~44.0 g/mol. This
means that one mole (6.02 x 1023) of carbon dioxide molecules will
weigh about 44.0g.
+ 16.0 amu
18.0 amu
Find the formula mass of the following:
T.
1) H2
2) HCl
3) H2O
4) Ca(OH)2
S.
1) O2
2) CO2
3) NH3
4) Al(NO3)3
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II. Formula Conversion Problems - use dimensional analysis as problem-solving method:
A. One-Step Problems
T.
1) What is the mass of 1.00 moles of carbon
dioxide (CO2) molecules?
2) What is the mass of 3.30 moles of carbon
dioxide (CO2) molecules?
3) How many moles of iron atoms weigh 50.0g?
S.
1) What is the mass of 2.50 moles of Ni ?
g Ni
5.00mol Ni 

1mol Ni
2) How many moles of NaOH weigh 50.0g?
50.0 g NaOH 
1mol NaOH

g NaOH
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3) How many moles of sodium oxide (Na2O) weigh 3.54g?
T. Assume conditions are at STP:
Standard Temperature and Pressure (STP)
Standard Pressure = 1 atm = 101.3 kPa = 14.7 psi
Standard Temperature = 0oC = 273 K
1) How many moles of He gas occupy 44.8 L of space?
2) If you have 3.77 mole of H2 gas, what size container will it require?
S.
1) How many moles of CH4 gas occupy 67.2 L of space?
67.2 L CH 4 
1mol CH 4

L CH 4
2) What is the volume of 0.679 moles of Cl2 gas?
2.34mol Cl2 
L Cl2

1mol Cl2
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B. Two-Step Problems
T.
*) How many atoms are there in a copper sphere that weighs ________________ grams?
2) How many liters of helium are there in 3.00g of helium at STP?
3) At STP, how many liters would be occupied by 20.0g of carbon dioxide gas?
S.
1) 65.0 g N2O = ? L N2O
65.0 g N 2 O 
1mol N 2 O
L N 2O


g N 2 O 1mol N 2 O
2) At STP, what mass of neon gas will occupy a 50.0 L container?
1mol Ne
g Ne
50.0 L Ne 

L Ne
1mol Ne
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V. Percent Composition - tells you what the compound is made of
* Law of Definite Proportions - compounds are substances whose elements always combine in
the same proportion
Find the percent composition of the following - round your percentages to 3 sig figs
T.
1) H2O
2) Al(NO3)3
S.
1) CO2
2) BaCl2
_____ % C ______ % O
_____ % Ba ____ % O
3) CaSO4
4) (NH4)2CO3
____ % Ca ____ % S _____ % O
____ % N ____ % H
____ % C ____ % O
5) How many grams of calcium are there in 15.0g of calcium sulfate (CaSO4)?
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VII. Equation Conversion Problems
* remember discussion of meaning of equation coefficients:
N2(g) + 3 H2(g)  2 NH3(g)
1 molecule + 3 molecules  2 molecules
1 mole molecules + 3 moles molecules 
At STP: 1 L gas
CANNOT SAY:
1 g N2
(violate Cons. of Mass??)
2 moles molecules
+
3 L gas

2 L gas
+
3 g H2

2 g NH3
A. Mole-mole Problems (one-step)
* must have balanced equation to solve these types of problems
T.
1) How many moles of NH3 are formed from 4.56 moles of N2?
2) How many moles of H2 are required to form 0.512 moles of NH3?
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3) How many liters of nitrogen gas (at STP) will react with 6.04 x 10-3 liters of hydrogen gas?
S8(s) + 12 O2(g)  8 SO3(g)
S.
1) How many moles of sulfur are needed to produce 1.00 mole of sulfur trioxide?
2) How many moles of sulfur trioxide are produced from reacting 0.331 moles of oxygen?
3) How many liters of oxygen gas (at STP) are required to produce 21.4 liters of sulfur trioxide
gas?
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B. Mass-mass problems (3-step)
* remember, we can’t relate coefficients in balanced equation to mass, therefore we must
“go through” moles - see “lily pad” sheet
2 NaCl(s)  2 Na(s) + Cl2(g)
T.
1) If you start with 10.0g of NaCl, how many grams of Na should you end up with?
2) If you start with 25.0g of NaCl, how many liters of chlorine gas would be produced STP?
VIII. Percent Yield - a means of measuring the “success” of a reaction
* - Using stoichiometry, we are predicting the amount of a product we EXPECT. In a
real laboratory situation, however, the amount of product you get often differs. The
percent yield gives a numerical value which represents the percent of the amount
expected you got in the experiment.
% Yield =
Actual Amount
x100%
Expected Amount
T. If the experiment in problem #1 above yielded 3.02g of sodium, what is the percent yield?
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S. If you began did the reaction in problem #2 and it gave you 4.20 L of chlorine gas, what
is the percent yield?
2 Al2O3(s)  4 Al(s) + 3 O2(g)
S.
1) If you get 13.6 L of oxygen after this reaction, how many grams of aluminum oxide did you
start with?
2) If you need to produce 50.0g of aluminum for your model plane, how many grams of
aluminum oxide must you decompose?
3) How many grams of aluminum do you get after decomposing aluminum oxide if this reaction
yields 40.0 L of oxygen?
4) If you begin with 2.50g of aluminum oxide, how many liters of oxygen gas (at STP) should
you get? If you actually get 0.701 L of oxygen gas from this, what is the percent yield?