Using Stoichiometry

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!