How does the mean free path vary with the number density of a gas?

If the pressure in a closed vessel is reduced by drawing out some gas,the mean free path of the molecules

Mean free path of molecules in a polyatomic gas is independent of

Two ideal gas thermometers $A$ and $B$ use oxygen and hydrogen respectively. The following observations are made:

Temperature	Pressure thermometer $A$	Pressure thermometer $B$
Triple-point of water	$1.250 \times 10^{5} \; Pa$	$0.200 \times 10^{5} \; Pa$
Normal melting point of sulphur	$1.797 \times 10^{5} \; Pa$	$0.287 \times 10^{5} \; Pa$

$(a)$ What is the absolute temperature of the normal melting point of sulphur as read by thermometers $A$ and $B$?
$(b)$ What do you think is the reason behind the slight difference in answers of thermometers $A$ and $B$? (The thermometers are not faulty). What further procedure is needed in the experiment to reduce the discrepancy between the two readings?

The mean free path for a gas,with molecular diameter $d$ and number density $n$,can be expressed as:

The mean free path of a molecule of diameter $5\times10^{-10} \ m$ at the temperature $41^{\circ}C$ and pressure $1.38\times10^{5} \ Pa$ is given as . . . . . . $m$. (Given $k_{B}=1.38\times10^{-23} \ J/K$).