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# Modern vacuum pumps can evacuate a vessel down to pressure of $\;4.0 \times 10^{-15}\;atm\;$.at room temperature (300 K). Taking $\;R=8.3 JK^{-1}mole^{-1},1\;atm=10^{5}\;Pa\;$and $\;N_{Avogadro}=6 \times 10^{23}\;mole^{-1}\;$ the mean distance between molecules of gas in an evacuated vessel will be the order of :

$(a)\;0.2\;\mu m \qquad(b)\;0.2\;m m\qquad(c)\;0.2\;cm \qquad(d)\;0.2\;nm$