Academic ChemistryUnit 9

Stoichiometry

Essential Question:

How does stoichiometry relate to the principle of conservation of matter?

Vocabulary:

Composition Stoichiometry (275) Actual Yield (293) Limiting Reactant (288)Mole Ratio (276) Theoretical Yield (293) Excess Reactant (288)Reaction Stoichiometry (275) Percent Yield (293)

Textbook Reading and Homework:

9-1 Introduction to Stoichiometry pages 275-277 Questions # 1-39-2 Ideal Stoichiometric Calculations pages 280-287 Questions # 1ab,2a,3a 9-3 Limiting Reactants and Percent Yield pages 288-294 Questions # 1-2EOC’s Page 295 #2,7,10a,12ab,17a,22a,28a,33

Objectives:By the end of this unit you should…

1. Define Stoichiometry.2. Describe the importance of the mole ratio in stoichiometric calculations.3. Write a mole ratio relating two substances in a chemical equation.4. Calculate the amount in moles of a reactant or product from the amount of moles of a difference

reactant or product.5. Calculate the mass of a reactant or product from the mass of a different reactant or product.6. Describe a method for determining which of two reactants is a limiting reactant.7. Calculate the amount in moles of a product produced, given the amounts in moles of two

reactants, one of which is in excess.8. Calculate the mass in grams of a product produced, given the mass in grams of two reactants,

one of which is in excess.9. Distinguish between theoretical yield, actual yield, and percent yield.10. Calculate percent yield, given the actual yield and quantity of a reactant.

Activities and Labs:

Lab: Stoichiometry of S’moresLab: Stoichiometry Balloon RacesQuizzes throughout the Chapter (Converting Moles to Moles; Converting Mass to Mass; and % Yield)

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Mole review WorksheetCalculate the molar mass of the following, and then write the molar mass in ratio form.

1. KNO3

2. Na2SO4

3. Ca(OH)2

4. (NH4)2SO3

5. CuSO4

Moles and MassDetermine the number of moles in each of the quantities below. Use the factor label method.1. 25 g of KNO3

2. 125 g of Na2SO4

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3. 100. g of CuSO4

4. 74 g of Ca(OH)2

5. 35 g (NH4)2SO3

Determine the number of grams in each of the quantities below. Show all work.

1. 2.5 moles of KNO3

2. 0.50 moles of (NH4)2SO3

3. 1.70 moles of CuSO4

4. 0.25 moles of KCl

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Mole Ratio

1. For each of the following chemical reactions, balance the equation and write all possible mole ratios.

a. ____ HgO(s) → ________Hg(l) + _____ O2 (g)

b. ____ C3H8 + ____ O2 → ____ CO2 + ____ H2O

c. ____ Al (s) + ____ H2SO4 (aq) → ____ Al2(SO4)3(aq) + ____ H2(g)

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Mole-Mole Stoichiometry Problems

N2 + 3H2 → 2NH3

1. How many moles of hydrogen are needed to completely react with two moles of nitrogen?

Given:

Want:

Conversion:

2KClO3 →2KCl + 3O2

How many moles of oxygen are produced by the decomposition of six moles of potassium chlorate?

Given:

Want:

Conversion:

Zn + 2HCl → ZnCl2 + H2

How many moles of hydrogen are produced from the reaction of three moles of zinc with an excess of hydrochloric acid?

Given:

Want:

Conversion:

C3H8 + 5O2 → 3CO2 + 4H2O

How many moles of oxygen are necessary to react completely with four moles of propane (C3H8)?

Given:

Want:

Conversion:

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K3PO4 + Al(NO3)3 → 3KNO3 + AlPO4

2. How many moles of potassium nitrate are produced when two moles of

potassium phosphate react with excess aluminum nitrate?

Given:

Want:

Conversion:

Stoichiometry Conversions I:

Conversions of Quantities in Moles Use the factor label method

1. Ammonia, NH3, is widely used as a fertilizer and in many household cleaners. How many moles of ammonia are produced when 6 mol of hydrogen gas react with an excess of nitrogen gas?

N2 + H2 → NH3 (balance reaction first!)

Given:

Want:

Conversion:

2. The decomposition of potassium chlorate, KClO3, is used as a source of oxygen and potassium chloride in

the laboratory. How many moles of potassium chlorate are needed to produce 15 mol of oxygen?

KClO3 → KCl + O2 (balance reaction first!)

Given:

Want:

Conversion:

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3. The elements lithium and oxygen react explosively to form lithium oxide, Li2O. How many moles of lithium oxide will form if 2 mol of lithium react?

Li + O2 → Li2O (balance reaction first!)

Given:

Want:

Conversion:

4.Tetrachloroethylene, C2Cl4 is a dry-cleaning fluid made from acetylene, C2H2, and Cl2, according to this equation: C2H2 + 3Cl2 + Ca(OH) 2 → C2Cl4 + CaCl2 + 2H2O. How many moles of Cl2 are needed to react with 4 mol of C2H2?

Given:

Want:

Conversion:

5. The disinfectant hydrogen peroxide, H2O2, decomposes to form water and oxygen gas. H2O2 → O2 + H2O (balance reaction first!)

How many moles of O2 will result from the decomposition of 5 mol of hydrogen peroxide?

Given:

Want:

Conversion:

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Conversions of Amounts in Moles to Mass1. When magnesium burns in air, it combines with oxygen to form magnesium oxide according to the following

equation. 2Mg + O2 →2MgO. What mass in grams of magnesium oxide is produced from 2.00 mol of magnesium? Given:

Want:

Conversion:

2. What mass in grams of oxygen combines with 2.00 mol of magnesium in the above reaction?

Given:

Want:

Conversion:

3. What mass of glucose (C6H12O6) can be produced from a photosynthesis reaction that occurs using 10 mol CO2 in the following equation:

6CO2 + 6H2O→ C6H12O6 + 6O2.

Given:

Want:

Conversion:

1. When sodium azide, NaN3 is activated in an automobile airbag, nitrogen gas and sodium are produced according the equation: 2NaN3→ 2Na + 3N2 If 0.500 mol of NaN3 react, what is the mass in grams of nitrogen would result?

Given:

Want:

Conversion:

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4. Coal can be converted to methane gas by a process called coal gasification. The equation for the reaction is the following: 2C + 2H2O → CH4 + CO2. What mass in grams of carbon is required to react with water to form 1.00 mol CH4?

Given:

Want:

Conversion:

Stoichiometry Practice Problems II Conversions of Mass to Moles 1. Mercury (II) oxide when heated decomposes to oxygen gas and mercury.

HgO → O2(g) + Hg(s) (balance reaction first!)

a. How many moles of mercury(II) oxide, HgO, are needed to produce 125 g of oxygen,O2?

Given:

Want:

Conversion:

b. How many moles of mercury are produced?

Given:

Want:

Conversion:

2. Chlorine gas can be produced commercially by passing an electrical current through a concentrated solution of sodium chloride. The unbalanced equation is as follows:

NaCl + H2O → NaOH + Cl2 + H2

a. If there are 250 g of NaCl, how many moles of Cl2 can be produced?

Given:

Want:

Conversion:

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NaCl + H2O → NaOH + Cl2 + H2

c. How many moles of H2 can be produced?

Given:

Want:

Conversion:

4. The compound PtCl2(NH3)2 is effective as a treatment for some cancers. It is synthesized by the reaction shown in the following equation: K2PtCl4 + 2NH3 → 2KCl + PtCl2(NH3)2

a. How many moles of K2PtCl4 must react in order to produce 30.0 g of PtCl2(NH3)2?

Given:

Want:

Conversion:

b. How many moles of NH3 are needed to produce 30.0 g of PtC12(NH3)2?

Given:

Want:

Conversion:

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Mass-Mass Calculations

1. Nitrous oxide, N2O is sometimes used as an anesthetic in dentistry. It is produced in the following reaction: NH4NO3 → N2O + 2H2O

a. How many grams of NH4NO3 are required to produce 33.0 g of N2O?

Given:

Want:

Conversion:

b. How many grams of water are produced in this reaction?

Given:

Want:

Conversion:

2. When copper metal is added to silver nitrate in solution, silver metal and copper(II) nitrate are produced.

AgNO3 +Cu → Cu(NO3)2 + Ag (balance reaction first!)a. What mass of silver is produced from 100 g of Cu?

Given:

Want:

Conversion:

b. What mass of aluminum is produced by the decomposition of 5.0 g of Al2O3 into oxygen gas and aluminum metal? Al2O3 Al + O2 Balance First

Given:

Want:Page 14 of 22

Conversion:

3. Sodium peroxide reacts vigorously with water to produce sodium hydroxide and oxygen. The unbalanced equation is the following:

Na2O2 + H2O → NaOH + O2. (balance reaction first!)

a. What mass in grams of O2 is produced when 50.0 g of Na2O2 react?

Given:

Want:

Conversion:

b. What mass in grams of water is needed to react completely with the Na2O2?

Given:

Want:

Conversion:

4. How many grams of H2O can be made from 5.00 grams of oxygen?2H2 + O2 → 2 H2O

Given:

Want:

Conversion:

5. How many grams of water will be produced when 100.00 grams of H2S04 (98.0 g/mol) is reacted? 2 NaOH + H2SO4 → 2 H2O + Na2SO4

Given:

Want:

Conversion:

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6. How many grams of CO2 will be made when 25.0 grams of Al2O3 (101.96 g/mol) is produced?

Al2(CO3)3 → Al2O3 + 3 CO2

Given:

Want:

Conversion:

7. How many grams of KClO3 (123 g/mol) are needed to make 130.0 g of O2? 2 KClO3 → 2 KCl + 3 O2 Given:

Want:

Conversion:

8. How many grams of Fe3O4 (232 g/mol) are needed to get 75.0 grams of iron? Fe3O4 + 4 CO →3 Fe + 4 CO2

Given:

Want:

Conversion:

9. How many grams of H2 can be made from 10.0 g of aluminum? (balance first) HCl+ Al → AlCl3 + H2

Given:

Want:

Conversion:

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10. How many grams of phosphoric acid (98.0 g/mol) are needed to react with 50.0 g of NaOH? 3NaOH + H3PO4 → Na3PO4 + 3 H2O

Given:

Want:

Conversion:

11. How many grams of water are needed to react with 17.5g of aluminum sulfide (150.0 g/mol)? Al2S3 + 6 H20 → 3 H2S + 2 Al(OH) 3

Given:

Want:

Conversion:

12. How many grams of Ca3(AsO4)2 (398g/mol) can be made form 95.5 g of Ca(OH)2? (balance) Ca(OH)2 + H3AsO4 →Ca3(AsO4)2 + H2O

Given:

Want:

Conversion:

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STOICHIOMETRY: LIMITING REAGENTN2 + 3H2 → 2NH3

1. How many grams of NH3 can be produced from the reaction of 28 g of N2 and 25 g of H2?

How much of the excess reagent in Problem 1 is left over?

Mg + 2HCI → MgCl2 + H2

2. What volume of hydrogen at STP is produced from the reaction of 50.0g of Mg and 75 g of HCl?

3. Silver nitrate and sodium phosphate are reacted in equal amounts of 200.grams each. How many grams of silver phosphate are produced?

AgNO3 + NaPO4 → AgPO4 + 3NaNO3

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Percent Yield

Most reactions do not go to completion, and so the actual yield is less than the percent yield. The percent yield is calculated as

Percent yield =actual yield

theoretical yield

100%% YIELD = ACTUAL YIELD (this must be

measured!) *100 THEORETICAL YIELD (which is what you calculated)

In other words, ,ost reactions aren’t produced as efficiently as you predicted. Note that the actual, measured yield must always be less than the theoretical yield, and someone always has to tell you what the actual yield is.

1. Ammonia is produced by the reaction

3 H2(g) + N2(g) 2 NH3(g)

(a) If N2(g) is present in excess and 55.6 g of H2(g) reacts, what is the theoretical yield of NH3(g)?

Given:

Want:

Conversion:

(b) What is the percent yield if the actual yield of the reaction is 159 g of NH3(g)?

2. Methyl alcohol (wood alcohol), CH3OH, is produced via the reaction

CO(g) + 2 H2(g) CH3OH(l)

a. A mixture of excess H2(g) and 7.45 g CO(g) are allowed to react. What is the yield of CH3OH?

Given:

Want:

Conversion:

b. Suppose the actual yield is 7.52 g of CH3OH. What is the % yield?

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Limiting Reactants Practice Problems1. Some rocket engines use a mixture of hydrazine, N2H4, and hydrogen peroxide, H2O2,

as the propellant. The reaction is given by the following equation: (Balance!)N2H4 + 2H2O2 → N2 + 4H2O.

a. Which is the limiting reactant in this reaction when 0.750 mol of N2H4 is mixed with 0.500 mol of H2O2?

Reagent 1 (N2H4)

Given:

Want:

Conversion:

Reagent 2 (H2O2)

Given:

Want:

Conversion:

b. How much N2 can actually be produced?

c. Which is the reagent in excess?

d. Which is the limiting reagent?

e. How much of the excess reactant, in moles, remains unchanged? Given:

Want:

Conversion:

f. How much of each product in moles, is formed?

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1. Zinc and sulfur react to form zinc sulfide according to the following equation: Zn + S8 → ZnS

a. If 2.00 mol of Zn are heated with 1.00 mol of S8, identify the limiting reactant.

Reagent 1 (Zn)

Given:

Want:

Conversion:

Reagent 2 (S8)

Given:

Want:

Conversion:

c. How many moles of excess reactant remain?

d. How many moles of product are formed?

3. If 20.5g of chlorine is reacted with 20.5g of sodium, which reactant is in excess? How do you know?

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4. Over the years, the thermite reaction has been used for welding railroad rails and to ignite solid fuel rocket motors:

Fe2O3 + Al → Fe + Al2O3

a) If you have 25.0 g of Fe2O3 (and plenty of AI) , how many grams of Fe can you make?

b) If you have 18.4 g Al (and plenty of Fe2O3), how many g of Fe can you make?

c) What if you have 25.0 g of Fe2O3 and 18.4 g Al, how many grams of Fe can you make?

d) When the reaction is done, is there any Fe2O3 left over? How much?

e) Assume that you have 25.0 g of Fe2O3 and plenty of Al, how many grams of Al2O3 can you make, theoretically?

f) On the railway, the reaction doesn't proceed as well as you calculated (due to mixing problems, temperature and moisture effects, etc.). The actual amount of Fe made is 12.6 grams. What is the % yield of this reaction?

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