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For real gases, van der Waals equation is written as: (p + (an^2/v^2))(V - nb) = nRT 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:

1. (I) He < H2 < CO2 < O2, (II) CH4 > H2 > O2
2. (I) O2 < He < CO2 < H2, (II) H2 > O2 > CH4
3. (I) H2 < He < O2 < CO2, (II) CH4 > O2 > H2
4. (I) H2 < O2 < He < CO2, (II) O2 > CH4 > H2

Correct answer: (I) H2 < He < O2 < CO2, (II) CH4 > O2 > H2

Solution

The van der Waals constant 'b' is related to the size of gas molecules, so smaller molecules like He and H2 have lower 'b' values compared to larger molecules like CO2. The constant 'a' is related to intermolecular forces, so CH4 has the highest 'a' due to stronger dispersion forces, followed by O2 and then H2. Thus, the correct order is (I) H2 < He < O2 < CO2 and (II) CH4 > O2 > H2.

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