Native speakers would say:
Water boils at a temperature of 100 degrees Celsius. (Not "at the temperature of")Do native speakers ever use "at the temperature of"? If yes, in what context do they use the expression?
Similarly, native speakers would say:
Kobe has a population of 1.5 million.
Do they ever use "the population of" in the following case?
Kobe has a population of 1.5 million. The population of 1.5 million means that Kobe is a comparatively large city.
Answers to Q1
In expressing "weights and measurements", we normally use indefinite articles (in contrast to "possessive" sense, we use the) before an of-phrase. For example, a temperature of 20 Celsius degrees; a distance of 40 miles, a box of three tons, etc.
Answer to Q2
The population of 1.5 is an anaphoric reference(second mention) to the preceding "a population of 1.5 million(first mention).