Final answer:
The majority of interstellar gas in the Milky Way is located in the spiral arms and consists predominantly of hydrogen, with molecular clouds contributing significantly to the mass.
Step-by-step explanation:
Most of the interstellar gas in our galaxy is located in the spiral arms. About 99% of the interstellar medium is gas, with hydrogen being the chief constituent. The atomic hydrogen makes up the majority of the interstellar gas by mass and volume. The denser molecular clouds, which contain a good portion of the total mass of interstellar gas, are found primarily in these spiral arms as well. Interstellar gas can vary in temperature, from as cold as a few degrees above absolute zero to as hot as a million degrees, with such variations largely dependent on its proximity to hot stars and the impact of supernova explosions.