Helium gas (molar mass = 4.00 g·mol⁻¹) and sulfur hexafluoride gas (SF₆
CHEMISTRY · Multi-part · A Level · Chemistry · Amount of substance · Amount of substance
Problem
Helium gas (molar mass = 4.00 g·mol⁻¹) and sulfur hexafluoride gas (SF₆, molar mass = 146 g·mol⁻¹) are each held at 295 K in separate identical bulbs connected to a vacuum line that contains a small pinhole for effusion studies. The temperature and total pressures in both bulbs are the same.
Answer the following questions regarding the relative molecular motion of the two gases.
(a) Determine the numerical value of the ratio average speed of SF6moleculesaverage speed of He atoms at 295 K.
(b) Using your result from part (a), calculate the ratio of the effusion rates of He and SF₆ through the pinhole under the stated conditions.
(c) Suppose equal amounts (in moles) of He and SF₆ are placed in the same evacuated bulb and allowed to effuse simultaneously through the pinhole for a short, identical time interval. Predict which gas will show the larger fractional pressure drop in the bulb and justify your prediction.