Chemical Stoichiometry Stoichiometry - The study of quantities of materials consumed and produced in chemical reactions. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. Figure 3.1: (left) A scientist injecting a sample into a mass spectrometer. (right) Schematic diagram of a mass spectrometer. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. Atomic Masses Elements occur in nature as mixtures of isotopes Carbon = 98.89% 12C 1.11% 13C <0.01% 14C Carbon atomic mass = 12.01 amu Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. Figure 3.2: (a) Neon gas glowing in a discharge tube. (b) "peaks" and (c) a bar graph. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Figure 3.3: Mass spectrum of natural copper. When a sample of natural copper is vaporized and injected into a mass spectrometer, the results shown in Fig. 3.3 are obtained. Use these data to compute the average mass of natural copper. (The mass values for 63Cu and 65Cu are 62.93 amu and 64.93 amu, respectively.)

Copyright©2000 by Houghton Mifflin Company. All rights reserved. The Mole The number equal to the number of carbon atoms in exactly 12 grams of pure 12C. 1 mole of anything = 6.022  1023 units of that thing Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. Figure 3.4: Proceeding clockwise from the top samples containing one mole each of copper, aluminum, iron, sulfur, iodine, and (in the center) mercury. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

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Avogadro’s number equals 6.022  1023 units Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. React 1 Now Try This… Calculate the number of copper atoms in a 63.55 g sample of copper. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. Molar Mass A substance’s molar mass (molecular weight) is the mass in grams of one mole of the compound. CO2 = 44.01 grams per mole Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. QUESTION The compound Cr2O3 (chromium (III) oxide) is one of the key components responsible for the red color of ruby gems. If you had 34.8 grams of Cr2O3, how many grams of Chromium (atomic number = 24) metal would be present? 1. 11.9 grams of Cr 2. 5.85 grams of Cr 3. 23.8 grams of Cr 4. 69.8 grams of Cr Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. ANSWER Choice 3 correctly predicts the grams of Cr found in 34.8 grams of Cr2O3. 34.8 grams Cr2O3  1 mol/152 g = 0.229 moles of Cr2O3 0.229 Moles Cr2O3  2 mol Cr/1 mol Cr2O3 = 0.458moles of Cr 0.458 moles Cr  52.0 g/1 mol Cr = 23.8 grams OR, 2  (52.0)/152  100 = 68.4 % Cr 0.684  34.8 = 23.8 g Cr Section 3.3: Molar Mass Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. Percent Composition Mass percent of an element: For iron in iron (III) oxide, (Fe2O3) Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. React 5 Consider separate 100.0-gram samples of each of the following:  H2O N2O C3H6O2 CO2 Rank them from highest to lowest percent oxygen by mass. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. Formulas molecular formula = (empirical formula)n [n = integer] molecular formula = C6H6 = (CH)6 empirical formula = CH Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Empirical Formula Determination 1. Base calculation on 100 grams of compound. 2. Determine moles of each element in 100 grams of compound. 3. Divide each value of moles by the smallest of the values. 4. Multiply each number by an integer to obtain all whole numbers. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. QUESTION The dye indigo is a compound with tremendous economic importance (blue jeans wouldn’t be blue without it.) Indigo’s percent composition is: 73.27% C; 3.84% H; 10.68%N and 12.21% O. What is the empirical formula of indigo? 1. C6H4NO 2. C8H3NO 3. C8H5NO 4. I know this should be whole numbers for each atom, but I do not know how to accomplish that. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. ANSWER Choice 3 is the smallest whole number ratio of the atoms that make up a molecule of indigo. The percentage must be converted to a mass, then the mass is converted to moles of the atoms and finally, the smallest is divided into the others to obtain the proper ratio. Section 3.5: Determining the Formula of a Compound Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. Figure 3.6: Examples of substances whose empirical and molecular formulas differ. Notice that molecular formula = (empirical formula)n, where n is an integer. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Figure 3.7: The two forms of dichloroethane. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. Chemical Equations Chemical change involves a reorganization of the atoms in one or more substances. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. Chemical Equation A representation of a chemical reaction: C2H5OH + 3O2  2CO2 + 3H2O reactants products Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. Chemical Equation C2H5OH + 3O2  2CO2 + 3H2O 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©2000 by Houghton Mifflin Company. All rights reserved.

Calculating Masses of Reactants and Products 1. Balance the equation. 2. Convert mass to moles. 3. Set up mole ratios. 4. Use mole ratios to calculate moles of desired substituent. 5. Convert moles to grams, if necessary. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. Limiting Reactant The limiting reactant is the reactant that is consumed first, limiting the amounts of products formed. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Solving a Stoichiometry Problem 1. Balance the equation. 2. Convert masses to moles. 3. Determine which reactant is limiting. 4. Use moles of limiting reactant and mole ratios to find moles of desired product. 5. Convert from moles to grams. Copyright©2000 by Houghton Mifflin Company. All rights reserved.