Using Stoichiometry
Chemists use stoichiometry to predict amounts of reactants used and products formed in specific reactions
Always start with a balanced equation
• This tells you the correct amount of moles for each substance
Like a recipe
N₂ + 3H₂ → 2NH₃
Peanut Butter Cookies
1 cup butter 1 tsp. vanilla1 cup sugar 3 cups flour1 cup brown sugar 2 tsp. baking soda2 eggs ¼ tsp. salt1 cup peanut butter
makes 24 cookies
What if I need 48 cookies?What if I want only 12 cookies?What if I need 5 dozen?If 3 cups of peanut butter are used, how many cookies will I get?
2K + 2H₂O → 2KOH + H₂
• Potassium reacts with water to produce potassium hydroxide and hydrogen
• 2 mol potassium makes1 mol hydrogen
How much hydrogen is produced if only 0.0400 mol of potassium isused?
Review mole ratios
2K + 2H₂O → 2KOH + H₂
Which two ratios deal with potassiumand hydrogen?
Mole to Mole Conversion
1. balance equation2. Use correct mole ratio
Moles of known x Moles of unknown = Moles of Moles of known unknown
2K + 2H₂O → 2KOH + H₂
0.0400 mol K x 1 mol H₂ = 0.0200 mol H₂ 2 mol K
Moles of known x Moles of unknown = Moles of Moles of known unknown
Try it!
• One disadvantage of burning propane (C₃H₈) is that carbon dioxide (CO₂) is one of the products. How many moles of CO₂ is produced when 10.0 mol of C₃H₈ is burned in excess oxygen in a gas grill?
What is first step?
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
• Use correct mole ratio
Moles of known x Moles of unknown = Moles of Moles of known unknown
Mole to Mass Conversion
• If you know the moles of a reactant and you want to know the mass of a product
Mole to Mass Conversion
1.Balance equation2.Use correct mole ratio3.Multiply by molar mass
What is the mass of NaCl when Na reacts with 1.25 mol Cl gas?
1. Balance the equation
2Na + Cl₂ = 2NaCl
2. Use correct mole ratio
1.25 mol Cl₂ x 2 mol NaCl = 2.50 mol NaCl 1 mol Cl₂
2Na + Cl₂ = 2NaCl
2.50 mol NaCl
3. Multiply by molar mass
When working in a lab, you will be working with grams
to measure out substances…
Mass to Mass Conversions
Mass to Mass Conversions
1. Balance equation2. Convert grams to moles of known
substance3. Use correct mole ratio4. Multiply by molar mass
MacGyver saves the day again!
• MacGyver offers to help some criminals break into an armored vehicle in order to save a life
Chemistry in Action!!!
• http://www.youtube.com/watch?v=fF6FUVxAVzI
Mass to Mass Conversion
•3Mg + Fe₂O₃ → 3MgO + 2Fe + lots of heat
How many grams of Mg does MacGyver need to make 5.0 g of Fe?
3Mg + Fe₂O₃ → 3MgO + 2Fe + heat
1. Balance equation 2. Convert grams to moles of known
substance
5.0 g Fe = ? Mol / molar mass Fe
5.0 g Fe = .0895 mol Fe
3. Use correct mole ratio
3Mg + Fe₂O₃ → 3MgO + 2Fe + heat
Moles of known x Moles of unknown = Moles of Moles of known unknown
0.134 mol Mg
4. Multiply by molar mass
0.134 x 24.305 = 3.26 g Mg
Review
Worksheet!