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$d. Why do we feel comfortable in expressing the densities of gases in the units of \mathrm{g} \mathrm{dm}^{-3} rather than \mathrm{g} \mathrm{cm}^{-3} a unit which is used to express the densities of liquids and solids.$

$(b) Compare the rates of diffusion of the following pairs of gases:(i) H_{2} and D_{0} :$

$Q15 Explain the following facts(iv) Water vapours do not behave ideally at 273 \mathrm{~K} .$

$Q8. Derive the units for gas constant R in general gas cquation: when energy is expressed in ergs.$

$Fill in the blanks.(i). The product PV has the S.I. unit of(ii). Eight grams each of \mathrm{O}_{2} and \mathrm{H}_{2} at 27^{\circ} \mathrm{C} will have total \mathrm{K} . \mathrm{E} in the ratio of(iii). Smell of the cooking gas during leakage from a gas cylinder is due to the property of of gases.(iv). Equal of ideal gases at the same temperature and pressure contain number of molecules.(v). The temperature above which a substance exists only as a gas is called$

$7. 1.40 \mathrm{dm}^{3} volume of a gas measured at a temperature of 27^{\circ} \mathrm{C} and a pressure of 900 torr was found to have mass 2.273 \mathrm{~g} . Calculate the molecular mass of the gas [Hint: Calculate " n " by the formula PV=nRT and then molecular mass] (33.72 amu) Ans.$

$Q21.a. The relative densities of two gases \mathrm{A} and \mathrm{B} are 1: 1.5 . Find out the volume of B which will diffuse in the same time in which 150 \mathrm{dm}^{3} of A will diffuse?$