You should view this in slideshow mode and make sure to enable active content if asked Copyright © Houghton Mifflin Company. All rights reserved.3–13–1

Chapter Three: STOICHIOMETRY

Copyright © Houghton Mifflin Company. All rights reserved.3–33–3 Chemical Stoichiometry Stoichiometry - The study of quantities of materials consumed and produced in chemical reactions.

Atomic Masses

Copyright © Houghton Mifflin Company. All rights reserved.3–53–5 Atomic Masses Elements occur in nature as mixtures of isotopes Carbon = 98.89% 12 C 1.11% 13 C <0.01% 14 C Carbon atomic mass = (abundace x mass) =12.01 amu

Example 1: When a sample of copper is vaporized and injected in a mass spectrometer, the results indicate that 69.09% of the atoms have a mass of 62.93amu while the rest have a mass of 64.93amu. Calculate the average atomic mass of copper –See answers in Chapter 1 notes key Copyright © Houghton Mifflin Company. All rights reserved.3–63–6

Moles, Molar Mass, and Percent Composition

Copyright © Houghton Mifflin Company. All rights reserved.3–83–8 The Mole The number equal to the number of carbon atoms in exactly 12 grams of pure 12 C. 1 mole of anything =  units of that thing

Copyright © Houghton Mifflin Company. All rights reserved.3–93–9 Atomic mass unit (amu)- the unit of mass for a single atom If the mass of an atom in amu = the mass of a mole of atoms in g, what is the conversion between amu and g?  (6.022x10 23 atoms)(x amu/atom)=x g  If x=1g then:  6.022x10 23 amu=1 g

Copyright © Houghton Mifflin Company. All rights reserved.3–10 Which of the following is closest to the average mass, in grams, of one atom of copper? a) g b) g c) g d) g e) X g React 1

Copyright © Houghton Mifflin Company. All rights reserved.3–11 What is the mass of six atoms of americium?

Copyright © Houghton Mifflin Company. All rights reserved.3–12 Let’s Think About It Is your answer reasonable? What are the units on the periodic table? React 1

Copyright © Houghton Mifflin Company. All rights reserved.3–13 Molar Mass The mass, in grams, of 1mole of a substance –Numerically equal to the average atomic mass in amu/molecule –Add the masses of the atoms in a compound Mass Moles # units –# units=atoms, molecules, formula units, etc. Molar Mass Avogadro’s #

Copyright © Houghton Mifflin Company. All rights reserved.3–14 Now Try This… Calculate the number of copper atoms in a g sample of copper. React 1

Copyright © Houghton Mifflin Company. All rights reserved.3–15 Which of the following g samples contains the greatest number of atoms? a) Magnesium b) Zinc c) Silver React 2

Copyright © Houghton Mifflin Company. All rights reserved.3–16 Rank the following according to number of atoms (greatest to least): a) g of silver b) 62.0 g of zinc c) 21.0 g of magnesium React 3

Copyright © Houghton Mifflin Company. All rights reserved.3–17 Consider separate gram samples of each of the following: H 2 O, N 2 O, C 3 H 6 O 2, CO 2 –Rank them from greatest to least number of number of molecules. React 4

Copyright © Houghton Mifflin Company. All rights reserved.3–18 Consider separate gram samples of each of the following: H 2 O, N 2 O, C 3 H 6 O 2, CO 2 –Rank them from greatest to least number of oxygen atoms. React 5

Percent Composition

Copyright © Houghton Mifflin Company. All rights reserved.3–20 Percent Composition Mass percent of an element: For iron in iron (III) oxide, (Fe 2 O 3 )

Copyright © Houghton Mifflin Company. All rights reserved.3–21 Consider separate gram samples of each of the following: H 2 O, N 2 O, C 3 H 6 O 2, CO 2 Rank them from highest to lowest percent oxygen by mass. React 5

Copyright © Houghton Mifflin Company. All rights reserved.3–22 Evaluate Your Answer Is it always true that a larger subscript for oxygen in a compound leads to a greater percent oxygen by mass in a compound?

Determining the Formula of a Compound

Copyright © Houghton Mifflin Company. All rights reserved.3–24 Formulas molecular formula = (empirical formula) n [n = integer] molecular formula = C 6 H 6 = (CH) 6 empirical formula = CH

Copyright © Houghton Mifflin Company. All rights reserved.3–25 Finding empirical Formulas Percent composition→mass composition→mole composition→mole ratio→empirical formula 100g sampleMolar mass Divide by smallest # Make subscipts whole #’s

Copyright © Houghton Mifflin Company. All rights reserved.3–26 Molecular Formulas To determine the molecular formula from the empirical formula, divide the given molecular mass by the molecular mass of the empirical formula –You must be given the actual molecular mass Acetylene has an empirical formula of CH. The experimentally determined molar mass is What is the molar mass of acetylene? – 26.04g/mol = g/mol –The empirical formula then gets multiplied by 2 –C2H2–C2H2

Determine the empirical and molecular formulas for a compound that gives the following percentages upon analysis: 71.65%Cl, 24.27% C, and 4.07%H. The molar mass is found to be 98.96g/mol Copyright © Houghton Mifflin Company. All rights reserved.3–27

If you have a 100g sample then 71.65%Cl would be 71.6gCl so Copyright © Houghton Mifflin Company. All rights reserved.3–28

Molecular formula Copyright © Houghton Mifflin Company. All rights reserved.3–29

Example 2: A white powder is found to contain 43.64%phosphorus and 56.36% oxygen by mass. The compound has a molecular mass of g/mol, what are the empirical and molecular formulas? Example 3: Caffeine contains 49.48%carbon, 5.15% hydrogen, 28.87% nitrogen, and 16.49% oxygen by mass and has a molar mass of 194.2g/mol. Determine the molecular formula of caffeine Copyright © Houghton Mifflin Company. All rights reserved.3–30

Chemical Equations

Copyright © Houghton Mifflin Company. All rights reserved.3–32 Balancing Chemical Equations

Copyright © Houghton Mifflin Company. All rights reserved.3–33 Chemical Equation A representation of a chemical reaction: C 2 H 5 OH + 3O 2  2CO 2 + 3H 2 O reactantsproducts

Copyright © Houghton Mifflin Company. All rights reserved.3–34 Chemical Equation C 2 H 5 OH + 3O 2  2CO 2 + 3H 2 O The equation is balanced. 1 mole of ethanol reacts with 3 moles of oxygen to produce 2 moles of carbon dioxide and 3 moles of water

Copyright © Houghton Mifflin Company. All rights reserved.3–35 Notice The number of atoms of each type of element must be the same on both sides of a balanced equation. Subscripts must not be changed to balance an equation. A balanced equation tells us the ratio of the number of molecules which react and are produced in a chemical reaction. Coefficients can be fractions, although they are usually given as lowest integer multiples. Trial and error is a valid method to balance a chemical equation.

Copyright © Houghton Mifflin Company. All rights reserved.3–36 Helpful Hints 1.Write elements in same order in reactants and products 2.Balance oxygen and hydrogen LAST 3.If you balance all elements but one and the last one won’t balance: Double the first substance and start over

Copyright © Houghton Mifflin Company. All rights reserved.3–37 Which of the following correctly describes the balanced chemical equation given below? There may be more than one true statement. If a statement is incorrect, explain what is incorrect about it. 4Al + 3O 2  2Al 2 O 3 –For every 4 atoms of aluminum that react with 6 atoms of oxygen, 2 molecules of aluminum oxide are produced. –For every 4 moles of aluminum that react with 3 moles of oxygen, 2 moles of aluminum (III) oxide are produced. –For every 4 grams of aluminum that react with 3 grams of oxygen, 2 grams of aluminum oxide are produced. React 6

Copyright © Houghton Mifflin Company. All rights reserved.3–38 Which of the following correctly balance the chemical equation given below? There may be more than one correct balanced equation. If a balanced equation is incorrect, explain what is incorrect about it. CaO + C  CaC 2 + CO 2 I. CaO 2 + 3C  CaC 2 + CO 2 II. 2CaO + 5C  2CaC 2 + CO 2 III. CaO + (2.5)C  CaC 2 + (0.5)CO 2 IV. 4CaO + 10C  4CaC 2 + 2CO 2 React 7

Copyright © Houghton Mifflin Company. All rights reserved.3–39 Which of the following are true concerning balanced chemical equations? There may be more than one true statement. –The number of molecules is conserved. –The coefficients tell you how much of each substance you have. –Atoms are neither created nor destroyed. –The coefficients indicate the mass ratios of the substances used. –The sum of the coefficients on the reactant side equals the sum of the coefficients on the product side. React 8

Copyright © Houghton Mifflin Company. All rights reserved.3–40 Example 4: Solid ammonium dichromate is ignited and produces solid chromium (III) oxide, nitrogen gas, and water vapor. Write and balance this equation

Example 5: Ammonia gas reacts with oxygen gas to form nitrogen monoxide and water vapor. Write and balance the equation Copyright © Houghton Mifflin Company. All rights reserved.3–41

Stoichiometric Calculations: Amounts of Reactants and Products

Copyright © Houghton Mifflin Company. All rights reserved.3–43 Stoichiometry Stoichiometry—Measurement and calculation of the amount of matter in a reaction MassMolesMole Ratio MolesMass ## Avagadro’s Number Molar Mass Balanced Equation Molar Mass Avagadro’s Number Given SubstanceUnknown Substance Volume Density Volume

Copyright © Houghton Mifflin Company. All rights reserved.3–44 Solid lithium hydroxide is used in space vehicles to remove exhaled carbon dioxide from the living environment by forming solid lithium carbonate and liquid water. What mass of gaseous carbon dioxide can be absorbed by 1.00kg of lithium hydroxide?

Copyright © Houghton Mifflin Company. All rights reserved.3–45

Example 6: Baking soda (sodium bicarbonate) is often used as an antacid. It neutralizes hydrochloric acid in the stomach and produces aqueous sodium chloride, liquid water and carbon dioxide gas. Milk of magnesia, Mg(OH) 2 is also used to neutralize hydrochloric acid and produces aqueous magnesium chloride and water Which of these is a more effective antacid against 1.00g of stomach acid? Copyright © Houghton Mifflin Company. All rights reserved.3–46

Copyright © Houghton Mifflin Company. All rights reserved.3–47 Limiting Reactants

Copyright © Houghton Mifflin Company. All rights reserved.3–48 Mixture of CH 4 and H 2 O Molecules

Copyright © Houghton Mifflin Company. All rights reserved.3–49 Methane and Water Reacting

Copyright © Houghton Mifflin Company. All rights reserved.3–50 To determine the limiting reactant: 1.Convert the amount of each reactant to moles 2.Find the moles (or grams) of product that each could produce. (you must convert each to the same product) 3.Whichever one produces fewer moles (or grams) of product is the limiting reactant You can then use the number of moles to determine the amount of products formed

Copyright © Houghton Mifflin Company. All rights reserved.3–51 Nitrogen gas can be prepared by passing gaseous ammonia over solid copper (II) oxide at high temperatures. The other products of the reaction are solid copper and water vapor. If a sample containing 18.1g NH 3 is reacted with 90.4g CuO, which is the limiting reactant? How much nitrogen gas will be collected, in grams?

Copyright © Houghton Mifflin Company. All rights reserved.3–52

Copyright © Houghton Mifflin Company. All rights reserved.3–53 Which of the following reaction mixtures would produce the greatest amount of product, assuming all went to completion? Each involves the reaction symbolized by the equation: 2H 2 + O 2  2H 2 O a) 2 moles of H 2 and 2 moles of O 2. b)2 moles of H 2 and 3 moles of O 2. c)2 moles of H 2 and 1 mole of O 2. d) 3 moles of H 2 and 1 mole of O 2. e) Each would produce the same amount of product. React 11

Copyright © Houghton Mifflin Company. All rights reserved.3–54 Notice We cannot simply add the total moles of all the reactants to decide which reactant mixture makes the most product. We must always think about how much product can be formed by using what we are given, and the ratio in the balanced equation.

Copyright © Houghton Mifflin Company. All rights reserved.3–55 Methane (CH 4 ) reacts with the oxygen in the air to produce carbon dioxide and water. Ammonia (NH 3 ) reacts with the oxygen in the air to produce nitrogen monoxide and water. What mass of ammonia would produce the same amount of water as 1.00 g of methane reacting with excess oxygen? React 12

Copyright © Houghton Mifflin Company. All rights reserved.3–56 Let’s Think About It We need to know: –How much water is produced from 1.00 g of methane and excess oxygen. –How much ammonia is needed to produce the amount of water calculated above. React 12

Copyright © Houghton Mifflin Company. All rights reserved.3–57 Practice Example 7: For the following unbalanced equation, tell me which reactant is limiting, the mass of the excess reactant left over and the mass of CO 2 produced if there are 5.00g of each reactant. CS 2 +O 2 →CO 2 +SO 2

Copyright © Houghton Mifflin Company. All rights reserved.3–58 Percent Yield Theoretical yield—the maximum amount of product which can be produced from a given amount of reactant –Calculated from the balanced equation using stoichiometry Actual yield—the amount of product actually produced when an experiment is performed –Determined experimentally (given in a problem)

Copyright © Houghton Mifflin Company. All rights reserved.3–59 Percent yield= actual x 100 theoretical When excess potassium chromate is added to a solution containing 0.500g of silver nitrate, solid silver chromate and potassium nitrate are formed. What is the theoretical yield of silver chromate? If 0.455g of silver chromate form, what is the percent yield? Why is percent yield never 100%

Copyright © Houghton Mifflin Company. All rights reserved.3–61 Example 8: Methanol (CH 3 OH) can be manufactured by reacting carbon monoxide and hydrogen gas. Suppose 68.5kg CO is reacted with 8.60kg H 2. What is the theoretical yield of methanol? If 3.57x10 4 g are produced, what is the percent yield of the reaction?