To determine which pairs have higher entropy, we need to consider the physical states and the number of molecules involved in each scenario. Entropy generally increases with an increase in the number of particles and/or an increase in the disorder or randomness of the system.
1 kg of H2O(L) at STP vs. 1 liter of H2O(L) at STP: In this case, both scenarios involve the same substance (water) at the same temperature and pressure (STP). The only difference is the amount of water present. Since both are in the liquid state, the number of molecules is the same in both cases (water molecules are closely packed in the liquid state). Therefore, the entropy is the same for both scenarios.
CH4(g) at STP vs. C2H6(g) at STP: In this case, we have different substances: methane (CH4) and ethane (C2H6). Both are gases at STP, and the number of molecules is different for each compound. Methane (CH4) has one carbon atom and four hydrogen atoms, while ethane (C2H6) has two carbon atoms and six hydrogen atoms. Since ethane has more atoms and more degrees of freedom, it has a higher number of microstates (ways the molecules can be arranged), resulting in higher entropy compared to methane.
Therefore, the pair with the higher entropy is: 2) C2H6(g) at STP (higher entropy) compared to CH4(g) at STP (lower entropy).