The rms speed of helium atom at 300 K is approximately (M_He = 4 g/mol):
A~ 137 m/s
B~ 1368 m/s
C~ 5000 m/s
D~ 14 m/s
Answer & Solution
Correct answer: B. ~ 1368 m/s
$v_{rms} = \sqrt{3RT/M_0} = \sqrt{3 \cdot 8.314 \cdot 300/0.004} = \sqrt{1.87 \times 10^6} \approx 1368$ m/s. (Helium escapes Earth's atmosphere over time because rms speed exceeds escape velocity for many atoms at higher altitudes.)
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