For real gases van der Waals equation is written as
$$\left( {p + {{a{n^2}} \over {{V^2}}}} \right)$$ (V $$-$$ nb) = n RT
where $$a$$ and $$b$$ are van der Waals constants. Two sets of gases are
(I) O2, CO2, H2 and He
(II) CH4. O2 and H2
The gases given in set-I in increasing order of b and gases given in set-II in decreasing order of $$a$$, are arranged below. Select the correct order from the following
A
(I) He < H2 < CO2 < O2 (II) CH4 > H2 > O2
B
(I) O2 < He < H2 < CO2 (II) H2 > O2 > CH4
C
(I) H2 < He < O2 < CO2 (II) CH4 > O2 > H2
D
(I) H2 < O2 < He < CO2 (II) O2 > CH4 > H2
Explanation
Van der Waal gas constant '$$a$$' represent intermolecular force of attraction of gaseous molecules and Van der Waal gas constant 'b' represent effective size of molecules . Therefore order should be
(I) H2 < He < O2 < CO2 (II) CH4 > O2 > H2
3
AIPMT 2012 Mains
MCQ (Single Correct Answer)
A certain gas takes three times as long to effuse out as helium. Its molecular mass will be
A
27 u
B
36 u
C
64 u
D
9 u
Explanation
According to Graham's law of diffusion
$$r \propto {1 \over {\sqrt d }} \propto {1 \over {\sqrt M }}$$
50 mL of each gas A and of gas B takes 150 and 200 seconds respectively for effusing through a pin hole under the similar conditions. If molecular mass of gas B is 36, the molecular mass of gass A will be