Page 252 - Schaum's Outline of Theory and Problems of Applied Physics
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CHAP. 20]                        KINETIC THEORY OF MATTER                             237



            The number of molecules in a mole of any substance is Avogadro’s number N, whose value is

                                                    23
                                       N = 6.023 × 10 molecules/mol
        The number of molecules in a sample of a substance is the number of moles it contains multiplied by N.



                                   Atom of carbon      Molecule of water (H 2 O)



                                     m = 12 u               m = 18 u


                                   Mole of carbon          Mole of water






                                   m = 12 g

                                                          m = 18 g

                                                 Fig. 20-2




        SOLVED PROBLEM 20.8
              Find the mass of (a) the water molecule H 2 O and (b) the ethyl alcohol molecule C 2 H 6 O. The atomic
              masses of H, C, and O are, respectively, 1.008, 12.01, and 16.00 u.
              (a)                           2(H) = (2)(1.008) u = 2.02 u
                                            1(O) = (1)(16.00) u = 16.00 u
                                                             18.02 u
                                    m(H 2 O) = (18.02 u)(1.66 × 10 −27  kg/u) = 2.99 × 10 −26  kg
              (b)                           2(C) = (2)(12.01) u = 24.02 u
                                            6(H) = (6)(1.008) u = 6.05 u
                                            1(O) = (1)(16.00) u = 16.00 u
                                                             46.07 u
                                   m(C 2 H 6 O) = (46.07 u)(1.66 × 10 −27  kg/u) = 7.65 × 10 −26  kg


        SOLVED PROBLEM 20.9
              How many H 2 O molecules are present in 1 kg of water?

                                                    mass of H 2 O
                                Molecules of H 2 O =
                                                mass of H 2 O molecule
                                                     1kg              25
                                              =              = 3.34 × 10 molecules
                                                2.99 × 10 −26  kg
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