Stoichiometry
Chapter 3
Chemistry II Honors
Atomic Mass
The Mole
The mole is a unit, similar to other units we use in our everyday
lives.
1 dozen = 12
1 gross = 144
1 ream = 500
23
1 mole = 6.022 x 10
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There are exactly 12 grams of carbon-12 in one
mole of carbon-12.
Atomic Mass
Avogadro’s Number
23
6.022 x 10 is called Avogadro’s
number in honor of the Italian chemist
Amadeo Avogadro (1776-1855).
Atomic Mass
The Mole
Sample Problem A
How many grams of lithium are in 3.50 moles of lithium?
Atomic Mass
The Mole
Sample Problem B
How many moles of oxygen are in 18.2 grams of oxygen?
Atomic Mass
The Mole
Sample Problem C
How many atoms of chlorine are in 3.500 moles of chlorine?
Atomic Mass
The Mole
Sample Problem D
How many atoms of neon are in 18.2 grams of neon?
Determining the Formula of a Compounds
Formulas
Empirical formula - the lowest whole number ratio of atoms in a
compound.
Molecular formula - the true number of atoms of each element in
the formula of a compound.
molecular formula = (empirical formula)n [n = integer]
molecular formula = C6H6 = (CH)6
empirical formula = CH
a.
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Determining the Formula of a Compounds
Formulas
Formulas for ionic compounds are ALWAYS empirical formulas
(lowest whole number ratio).
a.
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Formulas for molecular compounds MIGHT be empirical formulas
(lowest whole number ratio).
Determining the Formula of a Compound
Determining Empirical Formulas
1. Base calculations on 100 grams of the compound.
2. Determine the moles of each element in 100 grams of the
compound.
3. Divide each value of moles by the smallest of the values.
4. Multiply each number by an integer to obtain all whole numbers.
Determining the Formula of a Compound
Determining Empirical Formulas
Sample Problem E
Adipic acid is 49.32% C, 43.84% O, and 6.85% H by mass. What is the
empirical formula?
Determining the Formula of a Compound
AP Exam Practice
Sample Problem F
An experiment is performed to determine the empirical formula of a
copper iodide formed by direct combination of elements. A clean strip
of copper metal is weighed accurately. It is suspended in a test tube
containing iodine vapor generated by heating solid iodine, a white
compound forms on the strip of copper, coating it uniformly. The strip
with the adhering compound is weighed. Finally, the compound is
washed completely from the surface of the metal and the clean strip is
dried and reweighed.
Determining the Formula of a Compound
AP Exam Practice
Sample Problem F
Mass of clean copper strip
1.2789 g
Mass of copper strip and compound
1.2874 g
Mass of copper strip after washing
1.2748 g
A. State how you would use the data above to determine each of the
following. (Calculations not required.)
i. The number of moles of iodine that reacted
ii. The number of moles of copper that reacted
Determining the Formula of a Compound
AP Exam Practice
Sample Problem F
Mass of clean copper strip
1.2789 g
Mass of copper strip and compound
1.2874 g
Mass of copper strip after washing
1.2748 g
B. Explain how you would determine the empirical formula for the
copper iodide.
C. Explain how each of the following would affect the empirical formula
that could be calculated.
i. Some unreacted iodine condensed on the strip.
ii. A small amount of the white compound flaked off before weighing.
Determining the Formula of a Compound
Determining Molecular Formulas
1. Find the molar mass of the compound.
2. Divide the molar mass by the mass given by the empirical formula.
3. Multiply the empirical formula by this number to get the molecular
formula.
Determining the Formula of a Compound
Determining Empirical Formulas
Sample Problem G
The molecular mass of adipic acid is 146 g/mol. What is the
molecular formula?
Stoichiometric Calculations
Solving Stoichiometry Problems
1. Balance the equation.
2. Convert masses to moles.
3. Determine which reactant is limiting.
4. Use moles of limiting reactant and mole ratios to find moles of
desired products.
5. Convert from moles to grams.
Stoichiometric Calculations
Solving Stoichiometry Problems
Sample Problem H
6.50 grams of aluminum react with an excess of oxygen. How many
grams of aluminum oxide are formed?
4Al(s) + 3O2 (g) ⎯⎯
→ 2Al2O3 (s)
Stoichiometric Calculations
Limiting Reactants
The limiting reactant is the reactant that is consumed first, limiting the
amount of product formed.
Stoichiometric Calculations
Limiting Reactants
Sample Problem I
How many moles of NH3 can be formed from 3.0 moles of N2 and
6.0 mol of H2? Identify the limiting reactant.
N 2 (g) + 3H 2 (g) ⎯⎯
→ 2NH 3 (g)
Stoichiometric Calculations
Limiting Reactants
Sample Problem J
Suppose a solution containing 3.50 g of Na3PO4 is mixed with a
solution containing 6.40 g of Ba(NO3)2. How many grams of
Ba3(PO4)2 can be formed?
2Na3 PO4 ( aq ) + 3Ba ( NO3 )2 ( aq ) ⎯⎯
→ 6NaNO3 ( aq ) + Ba3 ( PO4 )2 ( s )
Stoichiometric Calculations
Percentage Yield
The quantity of product that is calculated to form when all of the
limiting reactant reacts is the theoretical yield.
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Numerous reasons why we don’t always get teh theoretical yield.
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Instead, we measure in lab the actual yield.
Stoichiometric Calculations
AP Exam Practice
Sample Problem K
Iron (III) oxide can be reduced with carbon monoxide according to the
following equation.
Fe2O3 ( s ) + 3CO ( g ) ⎯⎯
→ 2Fe ( s ) + 3CO2 ( g )
A 16.2 L sample of CO(g) at 1.50 atm and 200.ºC is combined with
15.39 g of Fe2O3(s).
a.How many moles of CO(g) are available for the reaction?
b.What is the limiting reactant for the reaction? Justify your answer.
c.How many moles of Fe(s) are formed in the reaction?
Stoichiometric Calculations
Percentage Yield
Sample Problem L
Adipic acid, H2C6H8O4, is made by the following reaction:
2C6 H 12 ( l ) + 5O2 ( g ) ⎯⎯
→ 2H 2C6 H 8O4 ( aq ) + 2H 2O ( l )
a. Assuming that you start with 25.0 g of cyclohexane, C6H12, and that it
is the limiting reactant, what is the theoretical yield of adipic acid?
b. If 33.5 g of adipic acid is obtained, what is the percentage yield?
Stoichiometric Calculations
AP Exam Practice
Sample Problem M
A compound containing the elements C, H, N, and O is analyzed. When
a 1.2359 g sample is burned in excess oxygen, 2.241 g of CO2(g) is
formed. The combustion analysis also showed that the sample
contained 0.0648 g of H.
a. Determine the mass, in grams, of C in the 1.2359 g sample of the
compound.
b. When the compound is analyzed for N content only, the mass
percent of N is found to be 28.84 percent. Determine the mass, in
grams, of N in the original 1.2359 g sample of the compound.
Stoichiometric Calculations
AP Exam Practice
Sample Problem M
A compound containing the elements C, H, N, and O is analyzed. When
a 1.2359 g sample is burned in excess oxygen, 2.241 g of CO2(g) is
formed. The combustion analysis also showed that the sample
contained 0.0648 g of H.
c. Determine the mass, in grams, of O in the original 1.2359 g sample of
the compound.
d. Determine the empirical formula of the compound.
Stoichiometric Calculations
AP Exam Practice
Sample Problem M
A compound containing the elements C, H, N, and O is analyzed. When
a 1.2359 g sample is burned in excess oxygen, 2.241 g of CO2(g) is
formed. The combustion analysis also showed that the sample
contained 0.0648 g of H.
A different compound, which has the empirical formula CH2Br, has a
vapor density of 6.00 g/L at 375 K and 0.983 atm. Using these data,
determine the following.
e. The molar mass of the compound.
f. The molecular formula of the compound.
Stoichiometric Calculations
AP Exam Practice
Sample Problem N
In first of two experiments, a student is assigned the task of determining
the number of moles of water in one mole of MgCl2*n H2O. The
student collects the data shown in the following table.
Mass of empty container
22.347 g
Initial mass of sample and container
25.825 g
Mass of sample and container after first heating
23.982 g
Mass of sample and container after second heating
23.976 g
Mass of sample and container after third heating
23.977 g
Stoichiometric Calculations
AP Exam Practice
Sample Problem N
In first of two experiments, a student is assigned the task of determining
the number of moles of water in one mole of MgCl2*n H2O. The
student collects the data shown in the following table.
a. Explain why the student can correctly conclude that the hydrate was
heated a sufficient number of times in the experiment.
b. Use the data above to
i. calculate the total number of moles of water lost when the sample
was heated, and
ii. determine the formula of the hydrated compound.
Stoichiometric Calculations
AP Exam Practice
Sample Problem N
In first of two experiments, a student is assigned the task of determining
the number of moles of water in one mole of MgCl2*n H2O. The
student collects the data shown in the following table.
c. A different student heats the hydrate in an uncovered crucible, and
some of the solid splatters out of the crucible. This splattering will
have what effect on the calculated mass of the water lost by the
hydrate? Justify your answer.
Stoichiometric Calculations
AP Exam Practice
Sample Problem N
In the second experiment, a student is given 2.94 g of a mixture
containing anhydrous MgCl2 and KNO3. To determine the percentage
by mass of MgCl2 in the mixture, the student uses excess AgNO3(aq) to
precipitate the chloride ion as AgCl(s).
d. Starting with the 2.94 g sample of the mixture dissolved in water,
briefly describe the steps necessary to quantitatively determine the
mass of the AgCl precipitate.
Stoichiometric Calculations
AP Exam Practice
Sample Problem N
In the second experiment, a student is given 2.94 g of a mixture
containing anhydrous MgCl2 and KNO3. To determine the percentage
by mass of MgCl2 in the mixture, the student uses excess AgNO3(aq) to
precipitate the chloride ion as AgCl(s).
e. The student determines the mass of the AgCl precipitate to be 5.48
g. On the basis of this information, calculate each of the following:
i. The number of moles of MgCl2 in the original mixture.
ii. The percent by mass of MgCl2 in the original mixture.