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energetic than others. Some of the faster-moving molecules in the liquid penetrate the SECTION 21.6 • Distribution of Molecular Speeds 657 Quick Quiz 21.8 Consider the qualitative shapes of the two curves in Figure 21.12, without regard for the numerical values or labels in the graph. Suppose you have 5 nitrogen molecules and container B has 10 5 hydrogen molecules. The correct qualitative matching be- tween the containers and the two curves in Figure 21.12 is (a) container A corresponds Active Figure 21.12 The speed distribution function for 10 5 nitrogen molecules at 300 K and 900 K. The total area under either curve is equal to the total number of mol- ecules, which in this case equals 10 5 . Note that . v rms * v * v mp At the Active Figures link at http://www.pse6.com, you can set the desired tempera- ture and see the effect on the distribution curve. 200 160 120 80 40 200 400 600 800 1000 1200 1400 1600 T = 300 K Curves calculated for N = 10 5 nitrogen molecules T = 900 K N v , number of molecules per unit speed inter val (molecules/m/s) v rms v v (m/s) v mp Example 21.5 A System of Nine Particles Nine particles have speeds of 5.00, 8.00, 12.0, 12.0, 12.0, (A) Find the particles’ average speed. Solution The average speed of the particles is the sum of (B) What is the rms speed of the particles? Solution The average value of the square of the speed is 12.7 m/s # v # (5.00 $ 8.00 $ 12.0 $ 12.0 $ 12.0 $ 14.0 $ 14.0 $ 17.0 $ 20.0) m/s 9 Hence, the rms speed of the particles is (C) What is the most probable speed of the particles? Solution Three of the particles have a speed of 12.0 m/s, mp is 12.0 m/s. 13.3 m/s v rms # √ v 2 # √ 178 m 2 /s 2 # 178 m 2 /s 2 # v 2 # (5.00 2 $ 8.00 2 $ 12.0 2 $ 12.0 2 $ 12.0 2 $ 14.0 2 $ 14.0 2 $ 17.0 2 $ 20.0 2 ) m 2 /s 2 9 |