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The square root of is called the root-mean-square (rms) speed of the molecules. From Equation 21.4 we find that the rms speed is (21.7) where M is the molar mass in kilograms per mole and is equal to mN A . This expression shows that, at a given temperature, lighter molecules move faster, on the average, than ! 3 kg/mol, have an average speed approximately four times that of oxygen molecules, whose molar mass is 32.0 % 10 ! 3 kg/mol. Table 21.1 lists the rms speeds for various molecules at 20°C. v rms # √ v 2 # √ 3k B T m # √ 3RT M v 2 SECTION 21.1 • Molecular Model of an Ideal Gas 645 Root-mean-square speed ▲ PITFALL PREVENTION 21.1 The Square Root of the Square? Notice that taking the square does not “undo” the square because we have taken an is because the squaring is done after the av- is not , but rather v rms . v v 2 v (v) 2 v 2 Molar mass v rms Gas (g/mol) at 20!C(m/s) H 2 2.02 1 902 He 4.00 1 352 H 2 O 18.0 637 Ne 20.2 602 N 2 or CO 28.0 511 NO 30.0 494 O 2 32.0 478 CO 2 44.0 408 SO 2 64.1 338 Table 21.1 Some rms Speeds Example 21.1 A Tank of Helium A tank used for filling helium balloons has a volume of 3 and contains 2.00 mol of helium gas at 20.0°C. Assume that the helium behaves like an ideal gas. (A) What is the total translational kinetic energy of the gas molecules? Solution Using Equation 21.6 with n # 2.00 mol and T # (B) What is the average kinetic energy per molecule? Solution Using Equation 21.4, we find that the average 7.30 % 10 3 J # K tot
trans # 3 2 nRT # 3 2 (2.00 mol)(8.31 J/mol&K)(293 K) What If? What if the temperature is raised from 20.0°C to 40.0°C? Because 40.0 is twice as large as 20.0, is the total Answer The expression for the total translational energy 6.07 % 10 ! 21 J # 1 2 mv 2 # 3 2 k B T #
3 2 (1.38 % 10 ! 23 J/K)(293 K) Quick Quiz 21.1 Two containers hold an ideal gas at the same temperature and pressure. Both containers hold the same type of gas but container B has twice the |