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Heat and Thermodynamics question

2019 · 9 Apr · Shift 1 · Q46
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Heat and Thermodynamics question

2019 · 9 Apr · Shift 1 · Q46

JEE MainPhysicsHeat and ThermodynamicsMCQ+4 / −1
For a given gas at 1 atm pressure, rms speed of the molecule is 200 m/s at 127°C. At 2 atm pressure and at 227°C, the rms speed of the molecules will be :
  1. A
    100 m/s
  2. B
    100 5\sqrt 55​ m/s
  3. C
    80 5\sqrt 55​ m/s
  4. D
    80 m/s
View written solutionFree

Correct answer: B

  1. Use the formula for rms speed

For an ideal gas, vrms=3RTMv_{\text{rms}} = \sqrt{\frac{3RT}{M}}vrms​=M3RT​​ So for a given gas, since RRR and MMM are constant, vrms∝Tv_{\text{rms}} \propto \sqrt{T}vrms​∝T​

This means rms speed depends only on absolute temperature, not on pressure.

  1. Convert temperatures to Kelvin

Initial temperature: T1=127∘C=127+273=400 KT_1 = 127^\circ C = 127 + 273 = 400\,KT1​=127∘C=127+273=400K

Final temperature: T2=227∘C=227+273=500 KT_2 = 227^\circ C = 227 + 273 = 500\,KT2​=227∘C=227+273=500K

  1. Relate the two rms speeds

Given: v1=200 m/sv_1 = 200\,\text{m/s}v1​=200m/s

Using v2v1=T2T1\frac{v_2}{v_1} = \sqrt{\frac{T_2}{T_1}}v1​v2​​=T1​T2​​​ we get v2=200500400v_2 = 200\sqrt{\frac{500}{400}}v2​=200400500​​

v2=20054v_2 = 200\sqrt{\frac{5}{4}}v2​=20045​​

v2=200⋅52v_2 = 200 \cdot \frac{\sqrt{5}}{2}v2​=200⋅25​​

v2=1005 m/sv_2 = 100\sqrt{5}\,\text{m/s}v2​=1005​m/s

  1. Match with options

1005 m/s100\sqrt{5}\,\text{m/s}1005​m/s corresponds to Option B.

  1. Comparison with stored answer

Stored correct answer: B

Our derived answer: B

So they agree.

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