Calculations with Chemical Formulas and Equations

1
Calculations with Chemical Formulas and Equations

• Chapter 3

2
1 MOLE 6.022 x 1023 anything
The Mole ?
3
Molecular Weight

• The number of grams in 1 MOLE of any element or
  compound
• H2O (2H x 1 g/H) (1O x 16 g/O)

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Molecular Weight

• The number of grams in 1 MOLE of any element or
  compound
• H2O (2H x 1 g/H) (1O x 16 g/O) 18
  g/mole

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Practice--Determine the Molecular Weights of the
Following Compounds

• HCl
• NaOH
• C2H6O

6
Practice--Determine the Molecular Weights of the
Following Compounds

• HCl - 36.46 g/mole
• NaOH - 40.00 g/mole
• C2H6O - 46.08 g/mole

7
Percent Composition

• By burning or otherwise volatilizing a sample, we
  can determine the of an element present.

atomic mass in molecule
x 100
molecular mass
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Percent Composition

• Determine the number of grams of any element
  based on 100 g of compound.

1.01 g H 36.46 g HCl/mol
x 100 2.7 H
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An Example

• What is the composition of hydrogen in H2O (l)?

10
An Example

• What is the composition of hydrogen in H2O (l)?

2 moles H x 1.0 g /mole H
x 100 11 H
1 mole H2O x 18 g /mole H2O
11
Another Problem

• An unknown sample of a compound contains 6.444 g
  of B and 1.803 g of H. If the molecular weight
  of the compound is about 30g, what is the
  empirical formula?

number of moles of B
number of moles of H
12
Another Problem

• An unknown sample of a compound contains 6.444 g
  of B and 1.803 g of H. If the molecular weight
  of the compound is about 30 g, what is the
  empirical formula?

number of moles of B
1 mole
6.444g B
x
0.596 moles
10.81 g
number of moles of H
1 mole
1.803g H
x
1.785 moles
1.01 g
13
Another Problem

• An unknown sample of a compound contains 6.444 g
  of B and 1.803 g of H. If the molecular weight
  of the compound is about 30 g, what is the
  empirical formula?

1.785 moles H
2.995 3
0.596 moles B
BH3
B2H6
B3H9
14
Another Problem

• An unknown sample of a compound contains 6.444 g
  of B and 1.803 g of H. If the molecular weight
  of the compound is about 30 g, what is the
  empirical formula?

1.785 moles H
2.995 3
0.596 moles B
BH3
B2H6
B3H9
15
Quantitative Information from Chemical Equations

• A balanced chemical equation can tell you
  everything you need to know about the mass of
  each element in a reaction.

C3H8 (g) O2 (g)
CO2 (g) H2O (l)
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Quantitative Information from Chemical Equations

• A balanced chemical equation can tell you
  everything you need to know about the mass of
  each element of the reaction.

C3H8 (g) 5O2 (g)
3CO2 (g) 4H2O (l)
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Plan of Attack

• Balance chemical equation
• What do you know?
• Convert it to moles
• Determine from stoichiometry the moles of the
  unknown
• Convert to appropriate quantity

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Example

• Iron metal is obtained by reacting hematite
  (Fe2O3) with carbon monoxide, CO, in a blast
  furnace, according to the general reaction
• Fe2O3(s) CO(g) ---gt Fe(s) CO2(g)
• How many grams of iron can be produced from 1.00
  kg Fe2O3 ?

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Balance the Equation

• Fe2O3(s) CO(g) ---gt Fe(s) CO2(g)

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Balance the Equation

• Fe2O3(s) 3CO(g) ---gt 2Fe(s) 3CO2(g)

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Balance the Equation

• Fe2O3(s) 3CO(g) ---gt 2Fe(s) 3CO2(g)

Convert known mass to moles
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Balance the Equation

• Fe2O3(s) 3CO(g) ---gt 2Fe(s) 3CO2(g)

Convert known mass to moles
1.00 x 103g Fe2O3 x 1 mol Fe2O3 x
60g Fe2O3
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Balance the Equation

• Fe2O3(s) 3CO(g) ---gt 2Fe(s) 3CO2(g)

Convert known mass to moles
1.00 x 103g Fe2O3 x 1 mol Fe2O3 x
60g Fe2O3
Convert to unknown moles
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Balance the Equation

• Fe2O3(s) 3CO(g) ---gt 2Fe(s) 3CO2(g)

Convert known mass to moles
1.00 x 103g Fe2O3 x 1 mol Fe2O3 x
60g Fe2O3 2 mol Fe
x 55.8g Fe 1 mol Fe2O3 1 mol Fe
Convert to unknown moles
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Balance the Equation

• Fe2O3(s) 3CO(g) ---gt 2Fe(s) 3CO2(g)

Convert known mass to moles
1.00 x 103g Fe2O3 x 1 mol Fe2O3 x
60g Fe2O3 2 mol Fe x
55.8g Fe 1 mol Fe2O3 1 mol Fe 698 g
Convert to unknown moles
AND SOLVE
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Combustion Stoichiometry

• How many grams of oxygen are needed to completely
  combust 12.6 g of propane?

C3H8 (g) 5O2 (g)
3CO2 (g) 4H2O (l)
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Combustion Stoichiometry
How many grams of oxygen are needed to completely
combust 12.6 g of propane?
C3H8 (g) 5O2 (g)
3CO2 (g) 4H2O (l)
1 mole prop.
12.6 g propane x
0.287 moles
44.1 g propane
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Combustion Stoichiometry

• How many grams of oxygen are needed to completely
  combust 12.6 g of propane?

C3H8 (g) 5O2 (g)
3CO2 (g) 4H2O (l)
1 mole prop.
12.6 g propane x
0.287 moles
44.1 g propane
5 moles O2
0.287 moles propane x
x 32.0 g/mole O2
1 mole prop.
45.7 g O2
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What If We Only Had 40 g?

• We are out of luck, the best thing we can do is
  calculate the percent yield. Mass
  formed/mass expected x 100
• Oxygen would be the limiting reagent!

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EXAMPLE

• Methanol, CH3OH, is prepared industrially by the
  following gas-phase reaction. In a laboratory
  test, a reaction vessel was filled with 35.4 g CO
  and 10.2 g H2. Which reactant is limiting?
• CO(g) H2(g) CH3OH(g)

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EXAMPLE

• Methanol, CH3OH, is prepared industrially by the
  following gas-phase reaction. In a laboratory
  test, a reaction vessel was filled with 35.4 g CO
  and 10.2 g H2. Which reactant is limiting?
• CO(g) H2(g) ---gt CH3OH(g)
• BALANCE THE EQUATION

32
EXAMPLE

• Methanol, CH3OH, is prepared industrially by the
  following gas-phase reaction. In a laboratory
  test, a reaction vessel was filled with 35.4 g CO
  and 10.2 g H2. Which reactant is limiting?
• CO(g) 2H2(g) ---gt CH3OH(g)
• BALANCE THE EQUATION

33
EXAMPLE

• Methanol, CH3OH, is prepared industrially by the
  following gas-phase reaction. In a laboratory
  test, a reaction vessel was filled with 35.4 g CO
  and 10.2 g H2. Which reactant is limiting?
• CO(g) 2H2(g) ---gt CH3OH(g)
• BALANCE THE EQUATION

35.4 g CO x 1mole CO 1.26 M
28.01 g CO
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EXAMPLE

• Methanol, CH3OH, is prepared industrially by the
  following gas-phase reaction. In a laboratory
  test, a reaction vessel was filled with 35.4 g CO
  and 10.2 g H2. Which reactant is limiting?
• CO(g) 2H2(g) ---gt CH3OH(g)

?
1.26 M
5.06 M
35
EXAMPLE

• Methanol, CH3OH, is prepared industrially by the
  following gas-phase reaction. In a laboratory
  test, a reaction vessel was filled with 35.4 g CO
  and 10.2 g H2. Which reactant is limiting?
• CO(g) 2H2(g) ---gt CH3OH(g)

?
1.26 mol
5.06 mol
- 1.26 - 2(1.26) 1.26
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EXAMPLE

• Methanol, CH3OH, is prepared industrially by the
  following gas-phase reaction. In a laboratory
  test, a reaction vessel was filled with 35.4 g CO
  and 10.2 g H2. Which reactant is limiting?
• CO(g) 2H2(g) ---gt CH3OH(g)

?
1.26 mol
5.06 mol
- 1.26 - 2(1.26) 1.26
0 2.54 mol 1.26 mol