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Chapter 3: Stoichiometry

Chapter 3: Stoichiometry. Atomic Mass Moles and Molar Mass Percent Composition of Compounds Empirical & Molecular Formulas of Compounds Writing and Balancing Chemical Equations Stoichiometry Limiting Reactants Reaction Yields. Mass Spectrometer (Figure 3.1).

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Chapter 3: Stoichiometry

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  1. Chapter 3: Stoichiometry • Atomic Mass • Moles and Molar Mass • Percent Composition of Compounds • Empirical & Molecular Formulas of Compounds • Writing and Balancing Chemical Equations • Stoichiometry • Limiting Reactants • Reaction Yields

  2. Mass Spectrometer (Figure 3.1)

  3. Mass Spectrometer Signal for Neon(Figure 3.2)

  4. Example 1 The element rhenium (Re) has only two naturally occurring isotopes, 185Re and 187Re, with an average atomic mass of 186.207 amu. The atomic mass of 185Re is 184.953 amu and the atomic mass of 187Re is 186.956 amu. Find the natural abundance of each isotope. Answer: 62.6% 187Re and 37.4% 185Re

  5. Example 2 • What is the mass of 1.00 mole of methane, CH4? • Answer: 16.0 g

  6. Example 3 • How many moles of K2SO4 are present in 10.0 g of this substance? • Answer: 0.0574 moles of K2SO4

  7. Example 4 • How many moles of O atoms are present in 5.40 g of H2O2? How many O atoms is that? • Answer: 0.318 moles of O; 1.91x1023 atoms of O

  8. Example 5 • What is the mass of 2.75 moles of H3PO4? • Answer: 269 g

  9. Example 6 • What is the mass of one helium atom, on average, in grams? • Answer: 6.647x10-24 g

  10. Equipment for Combustion Analysis of a Compound (Figure 3.5)

  11. Example 7 Find the percent composition of acetic acid. Answer: 6.71% H, 40.00% C, 53.29% O

  12. Example 8 An unknown compound of C, H, and O is analyzed by combustion. A sample weighing 10.68 mg was burned completely, producing 16.01 mg of CO2 and 4.37 mg of H2O. Find the percent composition of this compound. Answer: 40.92% C, 54.50% O, 4.58% H

  13. Example 9 When 0.847 g of isobutylene (a hydrocarbon) is burned completely, 2.657 g of CO2 and 1.089 g of H2O were produced. Find the empirical formula of isobutylene. Answer: CH2

  14. Example 10 An unknown compound of C, H, and O is analyzed by combustion and found to contain 53.3% O, 40.0% C, and 6.72% H by mass. What is the empirical formula of this substance? Answer: CH2O

  15. Example 11 Menthol, a flavoring agent obtained from peppermint oil, contains only carbon, hydrogen and oxygen.  Combustion of 1.00 g of menthol yields 1.161 g of H2O and 2.818 g of CO2.  What is the empirical formula of menthol?  If the molecular mass of menthol is determined by mass spectrometry to be 156 amu, what is the molecular formula of menthol?

  16. Example 12 If the empirical formula of an unknown hydrocarbon is found to be CH and the molecular weight is 78.11 amu, what is the molecular formula? Answer: C6H6

  17. Example 13 Balance the following chemical equations by inspection: CH4(g) + O2(g) CO2(g) + H2O (g) C2H6 (g) + O2(g) CO2(g) + H2O (g) Na3PO4(aq) + Ba(NO3)2(aq) NaNO3 (aq) + Ba3(PO4)2(s)

  18. Example 14 Given the following balanced equation: Fe2O3(s) + 3 CO (g) 2 Fe (s) + 3 CO2(g) How many grams of iron can be produced from 2.0 kg of Fe2O3 reacting with excess CO? Answer: 1.4 kg Fe

  19. Example 15 Given the following balanced equation: CO (g) + 2 H2 (g)  CH3OH(g) If 35.4 g of CO are combined with 10.2 g of H2, how many grams of CH3OH can be formed? Answer: 40.5 g of CH3OH are produced

  20. Example 16 Aspirin, C9H8O4, is synthesized by heating salicylic acid, C7H6O3, with acetic anhydride, C4H6O3. If 2.00 g of salicylic acid is heated with 4.00 g of acetic anhydride, what mass of aspirin can be produced and what mass of which reactant is leftover? Note that the only other reaction product is acetic acid, HC2H3O2. Answer: 2.61 g of aspirin is produced and 2.52 g of acetic anhydride is leftover.

  21. Example 17 If 10.0 g of carbon disulfide are combined with 10.0 g of chlorine, what mass of carbon tetrachloride is produced and what mass of which reactant is leftover? The balanced equation for the reaction is: CS2(g) + 3 Cl2(g) CCl4(l) + S2Cl2(g) Answer: 7.23 g of CCl4 produced and 6.4 g of CS2 leftover

  22. Example 18 If 25.0 g of NaBr and 25.0 g of Cl2 are combined, 12.1 g of NaCl are produced. What is the percent yield for this reaction? NaBr(aq) + Cl2(g) Br2(l) + NaCl(aq) [unbalanced] Answer: 85.2%

  23. Example 19 CS2 (g) + 3 O2 (g) CO2 (g) + 2 SO2 (g) When 15.0 g of CS2 and 35.0 g of O2 are mixed, they react with a 78.6% yield. What mass of SO2 is produced? Answer: 19.8 g of SO2

  24. Example 20 Mercury and bromine are both liquid elements. They react with each other to form mercury (II) bromide and no other products. What mass of mercury (II) bromide will be produced from the reaction of 5.00 mL of mercury with 5.00 mL of bromine assuming an 82.2% yield? The density of mercury is 13.5 g/mL and the density of bromine is 3.2 g/mL. Answer: 29.7 g HgBr2

  25. Example 21 A sample of unknown metal X weighs 5.000 g.  It combines with 1.330 L of O2 gas to form the oxide X2O.  If the density of the O2 gas used is 1.308 g/L, what is the identity of metal X? Answer: Na(s)

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