6 steps to solving Stoichiometry problems

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6 steps to solving Stoichiometry problems

• Define the objective
• Write down the balanced chemical equation
• Write down the conversion factors
• Molecular Mass
• Molar ratios
• Conversions to SI units
• Transform known values gt moles
• Apply conversion factors
• Reconvert moles back to desired units (SI)

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It all comes down to MOLES
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Mass-Mole relationship in Chemical Reaction
4
How much hydrogen is required to produce one ton
of ammonia?
5
Hydrogen Example

• Hydrogen can come from natural gas or petroleum
• Cost of processes using H2 are usually determined
  by the cost of the H2
• For the Production of Ammonia
• How much hydrogen is required to produce one ton
  of ammonia?

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6 steps to solving Stoichiometry problems

• 1. Define the objective
• 2. Write down the balance chemical equation
• 3. Write down the conversion factors
• 4. Transform known values gt moles
• 5. Apply conversion factors
• 6. Reconvert moles back to desired SI units

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Ammonia Example

• 1. Define the objective
• Amount of H2 in grams needed for 1 ton of NH3
• 2. Write down the balance chemical equation

• 3. Write down the conversion factors

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Common Conversion Scheme

• Mass of A gt moles of A

conversion factors
moles of B gt grams of B
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Ammonia example Step 3

• 3. Write down the conversion factors
• Start with molecular weights
• 1 mole N2 28.02 grams (2 x 14.01)
• 1 mole of H2 2.016 grams (2 x 1.008)
• 1 mole of NH3 17.03 grams (14.01 3 x 1.008)
• Conversions needed for SI units
• 1 ton 907 kg
• Stoichiometric (mole) ratios based on Balanced
  Eq.
• 3 moles H2 per 1 mole of N2
• 3 moles H2 per 2 moles of NH3
• 1 mole N2 per 2 moles of NH3

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Ammonia example Step 4

• 4. Transform known values gt moles
• Look for limiting reagents
• Use limiting reagent to determine the amount of
  product formed
• Key is if two known values are given
• assume excess of others if only one is given

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Factor Label Method

• 4. Transform known values gt moles
• Mass A gt Mole A

1 ton NH3
907 kg
Mole NH3
1000 g
mol NH3
5.326x104
1 kg
1 ton
17.03 g NH3
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Ammonia example Step 5

• 5. Apply conversion factors
• keep in mind defined objective
• amount in grams of H2
• Moles A gt Moles B

5.326x104 mol NH3
3 mol H2
mol H2
7.989x104
2 mol NH3
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Ammonia example Step 6

• 6. Reconvert moles back to desired SI units

7.989x104 mol H2
2.016 g H2
1.611x105 g H2
mol H2
1.611x105 g H2 161 kg H2
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Stoichiometry - Table method
Problem
907 kg
?
Equation
Formula Masses, g
28.02
2.016
17.03
3
2
Recipe, mol
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Stoichiometry - Table method
Problem
907 kg
?
Equation
Formula Masses, g
28.02
2.016
17.03
3
2
Recipe, mol
907 kg NH3
1 mol NH3
3 mol H2
2.016 g H2
17.03 g NH3
2 mol NH3
mol H2
161 kg H2
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Stoichiometry - Table method
Problem
907 kg
?
Equation
Formula Masses, g
28.02
2.016
17.03
3
2
Recipe, mol
907 kg NH3
1 mol NH3
3 mol H2
2.016 g H2
17.03 g NH3
2 mol NH3
mol H2
161 kg H2
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Practice in small groups4.2.1, 4.2.2