The pressure exerted by 6×1023 chlorine molecules, n of which will strike per second on a wall of area A at an angle 60∘ with the normal, if the mass of Cl2 molecules is m and speed of each Cl2 molecule is v, is given by :
A
mvnA
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B
2mvnA
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C
mnvA
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D
2mnvA
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Solution
The correct option is CmnvA Momentum of each chlorine molecule with which it strikes the wall normally =mvcosθ Each molecule rebounds with the same momentum. So, the change of momentum =mvcosθ–(–mvcosθ) =2mvcosθ If n molecules strike per second on the wall, then change in momentum per second, i.e., force =2mvcosθ×n and force per unit area =FA=2mnvcosθA Thus, pressure exerted on the wall is P=2mnvcosθA=2mnvcos60∘A=mnvA