The ratio of the vapor densities of two gases at a given temperature is $9: 8$, The ratio of the rms velocities of their molecule is (A) \(3: 2 \sqrt{2}\) (B) \(2 \sqrt{2}: 3\) (C) \(9: 8\) (D) \(8: 9\)

Short Answer

Expert verified
The ratio of the rms velocities of the two gases is \(\dfrac{v_{1_{rms}}}{v_{2_{rms}}} = \dfrac{2\sqrt{2}}{3}\), so the correct answer is (B) \(2 \sqrt{2}: 3\).

Step by step solution

01

Set up the ratios of vapor densities and rms velocities

We are given the ratio of vapor densities as \[\dfrac{\rho_1}{\rho_2} = \dfrac{9}{8}\] Our aim is to find the ratio of the rms velocities of the gases: \[\dfrac{v_{1_{rms}}}{v_{2_{rms}}}\]
02

Use the formulas to express the given ratio

Express the given ratio of vapor densities with the formula: \[\dfrac{\dfrac{P_1M_1}{RT_1}}{\dfrac{P_2M_2}{RT_2}} = \dfrac{9}{8}\] Since the temperatures and pressures are given to be equal for both gases, we can rewrite the equation as: \[\dfrac{M_1}{M_2} = \dfrac{9}{8}\]
03

Use the rms velocity formula to express the desired ratio

Express the desired ratio of rms velocities using the rms velocity formula: \[\dfrac{\sqrt{\dfrac{3R T_1}{M_1}}}{\sqrt{\dfrac{3R T_2}{M_2}}} = \dfrac{v_{1_{rms}}}{v_{2_{rms}}}\] Since the temperatures are equal, we can rewrite the equation as: \[\dfrac{\sqrt{\dfrac{M_2}{M_1}}}{\sqrt{\dfrac{M_1}{M_2}}} = \dfrac{v_{1_{rms}}}{v_{2_{rms}}}\]
04

Combine the equations and solve for the desired ratio

Using the ratio of molar masses from Step 2, we have: \[\dfrac{\sqrt{\dfrac{8}{9}}}{\sqrt{\dfrac{9}{8}}} = \dfrac{v_{1_{rms}}}{v_{2_{rms}}}\] Simplifying the expression gives: \[\dfrac{\sqrt{8}}{\sqrt{9}} = \dfrac{2\sqrt{2}}{3}\] So, the ratio of the rms velocities of the two gases is: \[\dfrac{v_{1_{rms}}}{v_{2_{rms}}} = \dfrac{2\sqrt{2}}{3}\] Therefore, the correct answer is (B) \(2 \sqrt{2}: 3\).

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