If van der Waals constant "a" for NH₃, H₂ and CO₂ are respectively 4.17, 0.244, 1.36 and 3.59, then which one of the following gases is most easily liquefied?

$1.$ ${\mathrm{NH}}_3$
$2.$ ${\mathrm{H}}_2$
$3.$ ${\mathrm{O}}_2$
$4.$ ${\mathrm{CO}}_2$

50 mL of each gas A and of gas B takes 200 and 150 s respectively for effusing through a pinhole under the similar conditions. If the molecular mass of gas B is 36, the molecular mass of gas A will be:

1. 96

2. 128

3. 32

4. 64

The energy absorbed by each molecule (A₂) of a substance is 4.4 x 10^-19 J and bond energy per molecule is 4.0 x 10^-19 J. The kinetic energy of the molecule per atom will be:

1. $2.0\times {10}^{-20}$ $J$

2. $2.2\times {10}^{-19}$ $J$

3. $2.0\times {10}^{-19}$ $J$

4. $4.0\times {10}^{-20}$ $J$

If a gas expands at a constant temperature, it indicates that:

1. Kinetic energy of the molecules decreases

2. The pressure of the gas increases

3. The kinetic energy of the molecules remains the same

4. The number of molecules of gas increases