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Sia

青州's Question
Chemistry
Posted 10 months ago

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It takes 161 s161 \mathrm{~s} for an unknown gas to effuse through a porous wall and 97 s97 \mathrm{~s} for the same volume of N2\mathrm{N}_{2} gas to effuse at the same temperature and pressure. What is the molar mass of the unknown gas?
\square g/mol\mathrm{g} / \mathrm{mol}
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Answer from Sia
Posted 10 months ago
Solution
1
Use Graham's Law of Effusion: Graham's Law states that the rate of effusion of a gas is inversely proportional to the square root of its molar mass
2
Set up the ratio: According to Graham's Law, Rate of effusion of unknown gasRate of effusion of N2=Molar mass of N2Molar mass of unknown gas\frac{\text{Rate of effusion of unknown gas}}{\text{Rate of effusion of } N_2} = \sqrt{\frac{\text{Molar mass of } N_2}{\text{Molar mass of unknown gas}}}. Since the rates are inversely proportional to the times taken to effuse, we can write 97161=28.02Munknown\frac{97}{161} = \sqrt{\frac{28.02}{M_{\text{unknown}}}}, where MunknownM_{\text{unknown}} is the molar mass of the unknown gas
3
Solve for the molar mass of the unknown gas: Squaring both sides of the equation gives us (97161)2=28.02Munknown\left(\frac{97}{161}\right)^2 = \frac{28.02}{M_{\text{unknown}}}. Solving for MunknownM_{\text{unknown}} gives us Munknown=28.02(97161)2M_{\text{unknown}} = \frac{28.02}{\left(\frac{97}{161}\right)^2}
4
Calculate the molar mass: After performing the calculation, we find the molar mass of the unknown gas
Answer
Munknown=28.02(97161)244.01 g/molM_{\text{unknown}} = \frac{28.02}{\left(\frac{97}{161}\right)^2} \approx 44.01 \text{ g/mol}
Key Concept
Graham's Law of Effusion
Explanation
Graham's Law relates the rates of effusion of two gases to their molar masses, allowing us to calculate the molar mass of an unknown gas when compared to a known gas.

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