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Stoichiometry: 3 Calculations with Chemical Formulas and Equations Visualizing Concepts 3.1 Reactant A = blue, reactant B = red Overall, 4 blue molecules + 4 red atoms 4 molecules Since 4 is a common factor, this equation reduces to equation (a). 3.2 Write the balanced equation for the reaction. 2H₂ + CO The combining ratio of CO is 2:1. If we have 8 molecules, 4 CO molecules are required for complete reaction. Alternatively, you could examine the atom ratios in the formula of but the balanced equation is most direct. 3.3 (a) There are twice as many o atoms as N atoms, so the empirical formula of the original compound is (b) No, because we have no way of knowing whether the empirical and molecular formulas are the same. NO₂ represents the simplest ratio of atoms in a molecule but not the only possible molecular formula. 3.4 The box contains 4 atoms and 16 H atoms, so the empirical formula of the hydro- carbon is 3.5 (a) Analyze. Given the molecular model, write the molecular formula. Plan. Use the colors of the atoms (spheres) in the model to determine the number of atoms of each element. Solve. Observe 2 gray C atoms, 5 white H atoms, 1 blue N atom, 2 red o atoms. C₂H₅NO₂ (b) Plan. Follow the method in Sample Exercise 3.9. Calculate formula weight in amu and molar mass in grams. 38