Stoichiometry Section 12.1

What is Stoichiometry? Study of quantitative relationships between amounts of reactants used and products formed. Based on the Law of Conservation of Mass (Mass of the Reactants = Mass of the Products) Ex. 4 Fe (s) + 3 O 2 (g)  2 Fe2O3 (s)

Ex. 2 Al (s) + 3 Br2 (l)  2 AlBr3 (s) Mole Ratios Ratio between the numbers of moles of any two substances in a balanced chemical equation. Ex. 2 Al (s) + 3 Br2 (l)  2 AlBr3 (s) Mole Ratios: 2 Al : 3 Br2 2 Al : 2 AlBr3 3 Br2: 2 AlBr3

Mole Ratio Practice What mole ratios can be written for the decomposition of potassium chlorate? 2 KClO3 (s)  2 KCl (s) + 3 O2 (g) Answer: 2 KClO3 : 2 KCl 2 KClO3 : 3 O2 2 KCl : 3 O2

Mole – Mole Conversions The problem gives you moles and asks for moles as an answer Balance the equation Label the known substance (given in the problem) and the unknown substance (substance being asked for in the problem) Determine the mole ratio that relates the known and unknown substances Multiply the known number of moles by the mole ratio

(H) Mole – Mole Conversion Practice 2 K (s) + 2 H2O (l)  2 KOH (aq) + H2 (g) How many moles of hydrogen are produced when .0400 mole of potassium is used? Write the balanced equation Given = .0400 mole K Unknown = moles of H2 3. Write the mole ratio: 1 mole H2 2 mole K Convert using: moles of known x moles of unknown moles of known Answer: .0400 x 1 mole H2 = .0200 mol H2 2 moles K

Stoichiometry Practice I (Mole-Mole Conversions) H2 + O2  H2O 8. How many moles of water can be produced with 6 moles of hydrogen? 9. How many moles of oxygen would be required to fully react with 8 moles of hydrogen? 10. How many moles of water can be produced with 4 moles of oxygen? 11. How many moles of hydrogen would be required to produced 10 moles of water? 12. How many moles of oxygen would be needed to produce 20 moles of water?

Mole – Mass Relationships Coefficients in a balanced equation can be interpreted in terms of representative particles and also by numbers of moles of particles. We can also use what we know about the conversion factor that relates mass and number of moles to find the mass of either the reactants or products.

How to solve mole to mass problems Problem gives you moles and asks for mass as an answer Balance equation Label the known substance and the unknown substance Determine the mole ratio that relates the known and unknown substances Multiply the known number of moles by the mole ratio Multiply the number of moles by the molar mass of the unknown substance

Mole – Mass Relationships Ex. 4 Fe (s) + 3 O2 (g)  2 Fe2O3 (s) 4 mol Fe x 56 g Fe = 224 g Fe 1 mol Fe 3 mol O2 x 32 g O2 = 96 g O2 1 mol O2 Mass of Reactants = 320 g 2 mol Fe2O3 x 160 g Fe2O3 = 320 g 1 mol Fe2O3 Mass of Products = 320 g

Mass to Mass Relationships in Equations Problem gives you mass and asks for mass as an answer Balance equation Label the known substance and the unknown substance Convert from the mass to the moles Use a mole ratio Use the molar mass to convert from moles to mass

(CP) Mole – Mole Relationships in Equations Example 2 H2 + O2  2 H2O How many moles of water can be produced with 6 moles of hydrogen? Equation is balanced. There are 2 moles of hydrogen, 1 mole oxygen, and 2 moles water. 6 H2 = x H2O 6 = x and 2x = 12 2 H2 2 H2O 2 2 x = 6 4. X = 6 moles water

(CP) Mole – Mole Relationships in Equations Balance the equation. Coefficients = number of moles of each substance. Use “x” for the “how many” compound. Use ratio of moles given in problem to actual moles in equation. Set up ratio. Solve.

(CP) Mole to Mass Relationships in Equations Balance the equation. Coefficients = # of moles of each substance. Use “x” for the “how many” compound. Use ratio of moles given in problem to actual moles in equation. Set up ratio and solve for “x”. Multiply your answer by the molar mass of the element or compound you are trying to find. (Example: Stoichiometry Practice I, # 4 at bottom)