Which substance has the lowest standard molar entropy at 25°c?

Full question:

First of all, bear in mind that this question is incomplete. Here is the full question:

Which substance has the lowest standard molar entropy at 25°C?

a) N₂O₄ (g)

b) He (g)

c) NO₂ (g)

d) NO (g)

Answer:

He has the lowest standard molar entropy (126.1 J/K. mol)

Explanation:

Standard entropy is the absolute entropy of a substance at 1 atm and 25°C.

Helium is a monatomic gas, which cannot execute rotational or vibrational motions, in contrast with the other gases of the list. We need to keep in mind that that the spinning motion of an atom about its own axis does not constitute a rotational motion because it does not displace the position of the nucleus.

The lowest standard molar entropy at 25°c for this gases (in J/K. mol) is:

N₂O₄ (g) : 304.3

He (g) : 126.1

NO₂ (g) : 240.46

NO (g) : 210.6