Bell Work: Mole Ratios 2 C 4 H 10 + ___ O 2 → ___ CO 2 + ___ H 2 O 1. Finish balancing the equation. How many moles of oxygen will react? 3. water will be produced? 4. butane will react? 5. carbon dioxide will be produced?

Stoichiometric Calculations Section 12.2

What is Stoichiometry? The study of the relationship between amounts of reactants used and products formed in a chemical reaction Based on the Law of Conservation of Mass and Matter Matter is neither created nor destroyed Mass of reactants equals the mass of the products

PROBLEM Interpret in terms of particles, moles, and mass. Show that mass is conserved: (Hint: look at coefficients for particles & moles) 4 Al + 3O 2  2 Al 2 O 3 Particles: Moles: Mass: Conserved?

SOLUTION: 4Al + 3O 2  2 Al 2 O 3 Particles: 4 molecules 3 molecule 2 molecule Moles: 4 mole 3 mole 2 moles Mass: 4 (27.0 g) + 3 (26.0 g) = 2 (102.0 g) Conserved? g = gYES! Law of Conservation of Matter shown.

Conservation of Mass Does the equation have conservation of mass? 4Fe (s) + 3O 2 (g)  2Fe 2 O 3 (s) 4Fe3O 2 2Fe 2 O 3 Molecules Moles Grams 4Fe3O 2 2Fe 2 O 3 Molecules 432 Moles Grams 4Fe3O 2 2Fe 2 O 3 Molecules 432 Moles 432 Grams 4Fe3O 2 2Fe 2 O 3 Molecules 432 Moles 432 Grams 4(55.8 g Fe) = g Fe 3(32.0 g O 2 ) = 96.0 g O 2 2(159.6 g Fe 2 O 3 )= g Fe 2 O 3 YES!

Using Stoichiometry In order to use stoichiometry correctly, you must start with a balanced chemical equation! The mole ratios needed come from the coefficients in a balanced chemical equation. First we will examine mole-to-mole conversions.

MOLE RATIO – is a ratio between the number of moles of any two substances in a balanced chemical equation There are three mole ratio pairs for the following: Ex. 4 Al(s) + 3 O 2 (g)  2 Al 2 O 3 (s) Note: 4 moles 3 moles 2 moles 4 mol Al and 3mol O 2 3 mol O 2 4 mol Al

MOLE RATIO – is a ratio between the number of moles of any two substances in a balanced chemical equation 4 mol Al and 2 mol Al 2 O 3 2 mol Al 2 O 3 4 mol Al 3 mol O 2 and 2 mol Al 2 O 3 2 mol Al 2 O 3 3 mol O 2 All stoichiometry calculations begin with a balanced equation and mole ratios!!

Using Stoichiometry Mole ratios can be used to convert the known number of moles of a substance to an unknown. Remember that when you use a conversion factor, the units must cancel out.

Mole-to-Mole Conversions Example: Sulfuric acid is formed when sulfur dioxide reacts with oxygen and water. If 12.5 mol sulfur dioxide reacts, how many mol sulfuric acid can be produced? How many mol oxygen are needed? First, write the balanced chemical equation. SO 2 + H 2 O + O 2  H 2 SO 4 Is this balanced? No… 2 SO 2 + 2H 2 O + O 2  2H 2 SO 4

Mole-to-Mole Conversions 2SO 2 + 2H 2 O + O 2  2H 2 SO 4 How much sulfuric acid is produced if 12.5 mol sulfur dioxide reacts ? Start with what you are given mol SO 2 x (mole-to-mole ratio)

Mole-to-Mole Conversions 2SO 2 + 2H 2 O + O 2  2H 2 SO 4 How much oxygen in needed if 12.5 mol sulfur dioxide reacts ? Start with what you are given mol SO 2 x (mole-to-mole ratio)

Mole-to-Mass Conversions Example: Determine the mass of sodium chloride produced when 1.25 moles of chlorine gas reacts vigorously with sodium. First, write a balanced chemical equation: 2Na (s) + Cl 2 (g)  2NaCl (s)

Example 2Na (s) + Cl 2 (g)  2NaCl (s) Write what is given. Use the mole ratio that relates mol NaCl to mol Cl 2. Determine the molar mass of NaCl. Molar mass NaCl = g/mol

Mass-to-Mass Conversions Example: Determine the mass of potassium chlorate needed to produce grams of oxygen. Write a balanced chemical equation: 2 KClO 3 (s)  2 KClO 2 (s) + O 2 (g)

Example 2 KClO 3 (s)  2 KClO 2 (s) + O 2 (g) Use the mole ratio that relates mol O 2 to mol KClO 3. 1 mol O 2 / 2 mol KClO 3 Determine the molar mass of KClO 3 and O 2. MM KClO 3 = g/mol and MM O 2 = 32.0 g/mol Start with what you are given.

Practice Problem (mass-to-mass) Pg. 362 #13

Steps in Stoichiometric Calculations 1) Write a balanced chemical equation, and interpret the equation in terms of moles. 2) Determine the moles of the given substance using a mass-to-mole conversion. 3) Determine the moles of the unknown substance from the moles of the substance given (mole-to- mole conversion). 4) From the moles of the unknown substance, determine the mass of the unknown substance using a mole-to-mass conversion.