Predict which substance in each pair has greater molar entropy. (1) no2(g) n2o4(g) (2) ch3och3(l) ch3ch2oh(l) (3) hcl(g) hbr(g)

Entropy is measure of disorder in system. Higher the disorder, greater the entropy.

Pair 1: NO2 (g) and N2O4 (g)
In the above case, entropy of N2O4 is more as compared to NO2, because N2O4 has more number of bonds as compared to NO2. So large number of vibrational energy levels are available for energy distribution which results in increasing entropy.

Pair 2: CH3OCH3(l) and CH3CH2OH(l)
Above compounds are structure isomers. They have same number of bonds and vibrational energy levels. However, presence of -OH group in CH3CH2OH results in intermolecular interaction via H bonding. This results in an ordered structure in CH3CH2OH as compared to  CH3OCH3. Due to this entropy in CH3CH2OH is lower as compared to CH3OCH3.

Pair 3: HCl(g) and HBr(g)
In present case, HBr will have higher entropy as compared to HCl, because of larger number of sub-atomic particles in Br. Also, the higher molecular mass of HBr favors larger entropy. 

They are all decomposition reactions.

It was ernest rutherford