Unit 6 Stoichiometry

What Exactly Is Stoichiometry? Composition stoich – deals with mass relationships of elements in compounds (review Ch 3) Reaction stoich – deals with relationships between reactants and products

What You’ve Learned So Far… Unit Conversions Naming Chemical Compounds Writing Chemical Formulas The Mole Writing Chemical Equations Balancing Chemical Equations

Mole Ratios Conversion factor relating amount of moles of any two substances

Example or

Molar Mass Ratios Al 2 O 3 – g/mol Al – g/mol O 2 – g/mol

Solving Stoichiometry Problems Must have: 1.Correct molecular formulas 2.Balanced chemical equation

Steps to Solving Problems 1.Write a balanced chemical equation 2.Identify information given in problem 3.Pick appropriate molar mass or mole ratio 4.Multiply numerators 5.Multiply denominators

Reaction Stoichiometry Mole-Mole Mole-Mass or Mass-Mole Mass-Mass

Type 1 Given quantity and unknown quantity in moles given Quantity given (in mol)  unknown Quantity unknown (in mol)

Stoichiometry Examples: Type 1 (mol-mol) Example 1: The combustion of 2.19 moles propane (C 3 H 8 ) produces how many moles carbon dioxide? Example 2: 41.8 mol lithium hydroxide is combined with carbon dioxide to produce lithium carbonate and liquid water. How much water is produced from this reaction?

Type 2 Given amount is in moles and unknown is mass in grams given Amount given (in mol)  unknown amount unknown (in mol)  unknown mass unknown (in g)

Stoichiometry Examples: Type 2 (mol-mass) Example 3: In photosynthesis, plants use energy from the sun to produce glucose C 6 H 12 O 6, and oxygen from the reaction of carbon dioxide and water. What mass, in grams, of glucose is produced when 3.00 mol of water react with carbon dioxide?

Type 3 Given is amount in grams and unknown is amount in moles given mass given (in g)  given amount given (in mol)  unknown amount unknown (in mol)

Stoichiometry Examples: Type 3 (mass-mol) NaHCO 3 and Mg(OH) 2 are both used as antacids – which is more effective per gram? Example 4: 1.00 g NaHCO 3 reacts with hydrochloric acid to produce sodium chloride, water, and carbon dioxide Example 5: 1.00 g Mg(OH) 2 reacts with hydrochloric acid to produce magnesium chloride and water

Stoichiometry Examples: Type 3 (mol-mass) Example 6 What mass of carbon dioxide, in grams, is needed to react with 3.50 mol of water in the photosynthetic reaction?

Objective III.A.3(i) – Use chemical equations to perform basic mole-mole, mass- mass, and mass-mole computations for chemical reactions

Type 4 Given is mass in grams and unknown is mass in grams given mass given (in g)  given amount given (in mol)  unknown amount unknown (in mol)  unknown mass unknown (in g)

Stoichiometry Examples: Type 4 (mass-mass) Example 7 Tin (II) fluoride is used in some toothpastes. It is made by the reaction of tin with hydrogen fluoride according to the following equation. Sn(s) + 2HF  SnF 2 (s) + H 2 (g) How many grams of SnF 2 are produced from the reaction of g of HF with Sn?

Limiting Reagents III.A.3(j) – identify limiting reagents and use this information when solving stoichiometry problems

What is a “limiting reagent”? The reactant that is completely used up first in a chemical reaction

How to Find the Limiting Reagent 1.Analyze the question 2.Identify the given information 3.Write a balanced chemical reaction 4.Convert given information of Reactant 1 to Product A 5.Convert given information of Reactant 2 to Product A

Theoretical Yield III.A.3(k) – compute theoretical yield, actual (experimental) yield, and percent yield III.A.3(l) – calculate percent error and analyze experimental errors that affect percent error

Theoretical Yield Using the limiting reagent to calculate the amount of product produced is called the theoretical yield This is how much product could be made if the limiting reactant was totally consumed

Experimental Yield Also called the actual yield The amount of product actually collected in the laboratory is called the experimental yield

Percent Yield

Example – Limiting Reactant Aluminum reacts with chlorine gas to form aluminum chloride. In a certain experiment, 10.0 g of aluminum is reacted with 35.0 g of chlorine gas. What mass of aluminum chloride will be produced, assuming a complete reaction? What mass of which reactant is left after the reaction?