rack up vs mop up

rack up is not a noun while mop up is a noun.

Word	Noun	Adjective	Verb	Adverb
rack up	No	No	Yes	No
mop up	Yes	No	Yes	No

As verbs, rack up and mop up are synonyms defined as:

Other synonyms of rack up include pip, whip, worst.

rack up (noun)	mop up (noun)
	a concluding action

rack up (verb)	mop up (verb)
supply a rack with feed for (horses or other animals)	finish a task completely
defeat thoroughly	defeat thoroughly
gain points in a game	to wash or wipe with or as if with a mop
place in a rack