is the adjective in this sentence empty: the car's gas tank is empty

Yes.

Also -- car's is a possessive noun and functions as an adjective
gas is also used as an adjective in this sentence.

Yes, the adjective in the sentence "the car's gas tank is empty" is "empty".

Yes, the adjective in the sentence "the car's gas tank is empty" is "empty."

To identify the adjective in this sentence, you can follow these steps:

1. Identify the noun that the adjective is modifying. In this case, the noun being modified is "gas tank."
2. Look for a word that describes or gives more information about that noun. In this sentence, "empty" is providing additional information about the gas tank.
3. Determine if the word is describing a characteristic or quality of the noun. In this case, "empty" describes the state of the gas tank.

Therefore, "empty" is the adjective modifying the noun "gas tank" in the sentence.