Question

Outside air generally contains some water vapor (typically 1-2% by mass). Does it take more, less, or the same heat to raise the temperature of 1kg moist air than is needed to raise the temperature of 1kg dry air by the same amount?

Answer 1

Since the molecular weight of water vapor is 18 and that of air is 29 (the N2/O2 weighted average), the presence of H2O vapor therefor lowers the average molecular weight of air. 1 kg of dry air will have fewer moles than 1 kg of moist air.

It will take more energy to heat the gas with more moles (moist air)

Water vapor has a higher molar specific heat (than O2 and N2) as well, but the fact that there will be more moles in the same mass of moist air is what makes the biggest difference