2009 Midterm

The unstable and stable isotopes have different

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Unformatted text preview: stable and stable isotopes have different chemical properties. E) Different isotopes of the same element may have different chemical properties; however, this is true only for heavy elements. 8. What is the mass of sulfur dioxide SO2 that contains the same number of sulfur atoms as 24.00 g of iron (II) disulfide FeS2? A) 12.81 g; B) 25.63 g; C) 51.26 g; D) 19.22 g; E) 38.44 g. The molecular mass of iron (II) disulfide is 119.99 amu, the molecular mass of sulfur dioxide is 64.07 amu. 24.00 g of it correspond to 24.00/119.99 = 0.2000 moles (4 significant figures). 0.2000 Moles of FeS2 contain 0.4000 moles of S. The same number of sulfur atoms will be found in 0.4000 moles of SO2, or in 0.4000*64.07 = 25.63 g (4 significant figures). 9. The mass percent of boron in boron (III) oxide is: A) 21.05%; B) 31.05%; C) 41.05%; D) 31.1%; E) 41.1% In one mole of boron (III) oxide B2O3 one finds 2 moles of B and 3 moles of O. The mass of one mole of B2O3 is 69.62 g. The mass of 2 moles of B is 10.81*2 = 21.62. The mass percentage of 21.62 = 0.3105 (4 significant figures) = 31.05% boron in B2O3 is 69.62 10. 1.00 mol of hydrogen gas is mixed with 42.8 litres of oxygen at a temperature of 298 K and a pressure 101 kPa and then the mixture is reacted to form steam. The gas constant is 8.31 J/mol.K. Which of the following statements is correct? A) the limiting reactant is hydrogen; B) the limiting reactant is oxygen; B) the limiting reactant is steam; D) both hydrogen and oxygen are limiting reactants; E) there are no limiting reactants. The balanced reaction is: 2 H2 (g) + O2 (g) → 2 H2O (g) To fully react with 1.00 mole of hydrogen gas, we need 0.500 moles of oxygen gas. In fact, we PV 101.3 ⋅ 10 3 Pa × 42.8 ⋅ 10 −3 m 3 = = 1.75 moles of oxygen gas (3 significant have nO2 = RT 8.31 J/mol.K × 298 K figures). Therefore, we do not have enough hydrogen, that is, hydrogen is a limiting reactant. Chemistry 1024b, Winter 2009 Midterm Exam with Solutions Page 3 out of 9 11. 42.6 g of sodium sulfate Na2SO4 were dissolved in 200 g of wa...
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