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Chemistry, The Central Science, 10th edition Theodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten Chapter 3 Stoichiometry: Calculations with Chemical Formulas and Equations Troy Wood University of Buffalo Buffalo, NY 2006, Prentice Hall The following equation is unbalanced: CS2 + O2 CO2 + SO2 What is the correct balanced equation? 1. 2. 3. 4. CS2 + 2 O2 2 CS2 + 12 O2 CS2 + 3 O2 CS2 + 6 O2 CO2 + 2 SO2 2 CO2 + 4 SO2 CO2 + 2 SO2 CO2 + 2 SO2 Correct Answer: 1. 2. 3. 4. CS2 + 2 O2 2 CS2 + 12 O2 CS2 + 3 O2 CS2 + 6 O2 CO2 + 2 SO2 2 CO2 + 4 SO2 CO2 + 2 SO2 CO2 + 2 SO2 This is the only case where the number of each type of atom is the same on both the reactant and product sides of the equation. Which of the following is the correct, balanced chemical equation for the combustion of ethanol (C2H5OH)? 1. 2. 3. 4. C2H5OH + O2 C2H5OH + 3 O2 C2H5OH + 6 O2 2 C2H5OH + 7 O2 CO2 + H2O 2 CO2 + 3 H2O 4 CO2 + 6 H2O 4 CO2 + 6 H2O O O C C O Correct Answer: 1. 2. 3. 4. C2H5OH + O2 C2H5OH + 3 O2 C2H5OH + 6 O2 2 C2H5OH + 7 O2 CO2 + H2O 2 CO2 + 3 H2O 4 CO2 + 6 H2O 4 CO2 + 6 H2O C OO O C O If the empirical formula of a compound is C2H2N but the molecular weight is 160 g/mol, what is the molecular formula? 1. 2. 3. 4. 5. C2H2N C4H4N2 C6H8N3 C8H8N4 None of the above choices is correct. Correct Answer: 1. 2. 3. 4. 5. C2H2N C4H4N2 C6H8N3 C8H8N4 None of the above choices is correct. FW C2H2N = (24 + 2 + 14) g/mol = 40 g/mol MW/FW = 160/40 = 4 Thus, molecular formula is 4 empirical formula 4(C2H2N) = C8H8N4 How many fluorine atoms are present in 2.0 moles of AlF3? 1. 2. 3. 4. 5. 3.0 1023 6.0 1023 1.2 1024 3.6 1024 None of the above answers is correct. Correct Answer: 1. 2. 3. 4. 5. 3.0 1023 6.0 1023 1.2 1024 3.6 1024 None of the above answers is correct. (2.0 moles AlF3)(6.0 1023 molecules AlF3/mole AlF3) (3 atoms F/1 molecule AlF3) = 3.6 1024 Which of the following samples has the largest number of atoms? 1. 2. 3. 4. 5. 28 g Si 28 g CO 28 g N2 15 g C2H4 All of the above have equal numbers of atoms. Correct Answer: 1. 2. 3. 4. 5. 28 g Si 28 g CO 28 g N2 15 g C2H4 All of the above have equal numbers of atoms. 28 g Si has 1 mole of atoms, whereas 28 g of CO and N2 have 2 moles of atoms, even though they also have only 1 mole substance. However, there are 6 atoms for every 1 mole of C2H4, so in this case there are slightly more than 3 moles worth of atoms. The molecular mass for the compound, XClO3, is 122.5 amu. The atomic mass of X is equal to ____ amu, which is the element ____. 1. 2. 3. 4. 5. 1.0; H 6.9; Li 23.0; Na 39.1; K 85.5; Rb Correct Answer: 1. 2. 3. 4. 5. 1.0; H 6.9; Li 23.0; Na 39.1; K 85.5; Rb MW = 122.5 amu = AW X + AW Cl + 3(AW O) 122.5 amu AW Cl 3 (AW O) = AW X 122.5 amu (35.45 amu) 3 (16.0 amu) = AW X 122.5 35.45 amu 48.0 amu = AW X AW X = 39.1 amu When the chemical equation, C4H10(g) + O2(g) H2O(g) + CO2(g), is balanced, the stoichiometric coefficients are, respectively: 1. 2. 3. 4. 1, 4, 5, 4 2, 13, 10, 8 2, 9, 8, 10 2, 7, 4, 5 Correct Answer: 1. 2. 3. 4. 1, 4, 5, 4 2, 13, 10, 8 2, 9, 8, 10 2, 7, 4, 5 The combustion of methane (CH4) is shown below: CH4(g) + 2 O2(g) 2 H2O(l) + CO2(g) How many grams of water are produced in the combustion of 1.0 moles of methane? 1. 2. 3. 4. 16 g 32 g 36 g 48 g Correct Answer: 1. 2. 3. 4. 16 g 32 g 36 g 48 g (1.0 mole CH4)(2 moles H2O/1 mole CH4) (18 g H2O/1 mole H2O) = 36 g 0.5 mol of carbon is combusted with 1.0 mol of oxygen gas. Which of the following statements is correct? 1. There is a molar excess of carbon, not all carbon is consumed. 2. There is a molar excess of oxygen, not all oxygen is consumed. 3. There are stoichiometrically equivalent amounts of carbon and oxygen gas. Correct Answer: 1. There is a molar excess of carbon, not all carbon is consumed. 2. There is a molar excess of oxygen, not all oxygen is consumed. 3. There are stoichiometrically equivalent amounts of carbon and oxygen gas. C and O2 react in a 1:1 stoichiometric ratio, therefore there must be excess O2 because there was a larger initial number of moles of it. 0.5 mol of carbon is combusted with 1.0 mol g of oxygen gas. How many moles of carbon dioxide are produced? 1. 2. 3. 4. 1.5 mol 1.0 mol 0.5 mol 2.0 mol Correct Answer: 1. 2. 3. 4. 1.5 mol 1.0 mol 0.5 mol 2.0 mol C and O2 react in a 1:1 stoichiometric ratio, therefore C must be the limiting reagent and O2 must be the excess reagent. Initially, (0.5 moles C)(1 mole CO2/1 mole C) = 0.5 mole CO2 24.0 g of carbon are combusted with 32.0 g of oxygen gas. How many grams of carbon dioxide are produced? 1. 2. 3. 4. 56.0 g 88.0 g 28.0 g 44.0 g Correct Answer: 1. 2. 3. 4. 56.0 g 88.0 g 28.0 g 44.0 g C and O2 react in a 1:1 stoichiometric ratio. Initially, (24.0 g C) (1 mole C/12.0 g C) (1 mole CO2/1 mole C) = 2.0 mole CO2 (32.0 g O2) (1 mole O2/32.0 g O2) (1 mole CO2/1 mole O2) = 1.0 mole CO2 Thus, O2 is limiting, and 1.0 mole of CO2 is 44.0 g. 3.0 moles Mg are combusted with 2.0 moles O2. What is the limiting reagent and how many moles of the excess reagent remain? 1. 2. 3. 4. Magnesium, 0.5 mole Magnesium, 1.0 mole Oxygen gas, 0.5 mole Oxygen gas, 1.0 mole Correct Answer: 1. 2. 3. 4. Magnesium, 0.5 mole Magnesium, 1.0 mole Oxygen gas, 0.5 mole Oxygen gas, 1.0 mole Mg and O2 react in a 2:1 stoichiometric ratio. Initially, (3 mole Mg)(2 mole MgO/2 mole Mg) = 1.5 mole MgO (2 mole O2)(2 mole MgO/1 mole O2) = 4 mole MgO Thus, Mg is limiting, and 3 mol Mg will consume 1.5 mol of O2, leaving 0.5 mole of O2.