At absolute zero, only one quantum state would represent the system, so the entropy is zero. As heat flows in, the multiplicity increases. Because entropy increases with the logarithm of the temperature, the rate of change (ΔS/ΔT) is proportional to 1/T. The molar entropy of water at zero Celsius equals the molar entropy of zero Celsius ice plus the heat of fusion divided by 273 Kelvin. That's what ΔS/ΔT tells us. The entropy of a gas can be calculated theoretically as a state function, depending on Temperature, Volume, and MW (or T, P and MW) or it can be determined experimentally by the addition of heat from temperatures close to 0 K through melting, vaporization to the final temperature.

The residual entropy of nitrous oxide at 0K is an interesting problem. Because the molecules in the crystal can be oriented in one of two ways, there is still entropy at absolute zero because of the random alignment of nitrous oxide molecules in the perfectly motionless lattice.