fifth vs 5th

fifth is a noun but 5th is not a noun.

Word	Noun	Adjective	Verb	Adverb
fifth	Yes	Yes	No	No
5th	No	Yes	No	No

As adjectives, fifth and 5th are synonyms defined as:

fifth and 5th: coming next after the fourth and just before the sixth in position

fifth (noun)	5th (noun)
the musical interval between one note and another five notes away from it
one part in five equal parts
a quantity of liquor equal to one fifth of a United States gallon
position five in a countable series of things

fifth (adjective)	5th (adjective)
coming next after the fourth and just before the sixth in position	coming next after the fourth and just before the sixth in position