Value of universal gas constant $(R)$ is same for all gases. What is its physical significance?
Unit of $R$ depends upon those units in which $p, V$ and $T$ are measured as, $R=\frac{p V}{n T}$. If pressure is measured in Pascal, per mole volume is measured in $\mathrm{m}^3$ and temperature is measured in Kelvin then units of ' $R$ ' are $\mathrm{Pam}^3 \mathrm{~K}^{-1} \mathrm{~mol}^{-1}$ or $\mathrm{Jmol}^{-1} \mathrm{~K}^{-1}$. Since, joule is the unit of work done, so ' $R$ ' is work done by the gas per mole per kelvin.
One of the assumptions of kinetic theory of gases states that "there is no force of attraction between the molecules of a gas." How far is this statement correct? Is it possible to liquefy an ideal gas? Explain.
This statement is correct only for ideal gases. It is not possible to liquefy an ideal gas because there is no intermolecular forces of attractions between the molecules of an ideal gas.
The magnitude of surface tension of liquid depends on the attractive forces between the molecules. Arrange the following in increasing order of surface tension:
Water, alcohol $\left(\mathrm{C}_2 \mathrm{H}_5 \mathrm{OH}\right)$ and hexane $\left.\left[\mathrm{CH}_3\left(\mathrm{CH}_2\right)_4 \mathrm{CH}_3\right)\right]$.
In the above given molecules, only hexane $\left(\mathrm{CH}_3\left(\mathrm{CH}_2\right)_4 \mathrm{CH}_3\right)$ is a non-polar molecule in which only Landon dispersion forces exist. These forces are very weak while both water and alcohol are polar molecules in which dipole-dipole interactions as well as H -bonding exists.
However, H -bonding interactions are much stronger in water than $\mathrm{H}_2 \mathrm{O}$, therefore, it possesses stronger intermo lecules than alcohol and hexane. Hence, the increasing order of surface tension is
$$\text { hexane }<\text { alcohol }<\text { water }$$
Greater is the attractive forces between the molecules, greater is the magnitude of surface tension of liquid.
Pressure exerted by saturated water vapour is called aqueous tension. What correction term will you apply to the total pressure to obtain pressure of dry gas?
Whenever a gas is collected over water, it is moist and saturated with water vapours which exert their own pressure. The pressure due to water vapours is called aqueous tension thus, the to tal pressure of the gas ( $p$ moist gas) is
$$p_{\text {moist gas }}=p_{\text {dry gas }}+\text { aqueous tension }$$
Thus, $p_{\text {dry gas }}$ is given as
$P_{\text {dry gas }}=P_{\text {moist gas }}-$ aqueous tension Hence, the correction term applied to the total pressure of the gas in order to obtain pressure of dry gas is $p$ moist gas - aqueous tension.
Name the energy which arises due to motion of atoms or molecules in a body. How is this energy affected when the temperature is increased?
The energy which arises due to motion of atoms or molecules in a body is known as thermal energy. It is a measure of average kinetic energy of the particles. It increases with increase in temperature.