mental capacity vs brain

mental capacity and brain both are nouns.

Word	Noun	Adjective	Verb	Adverb
mental capacity	Yes	No	No	No
brain	Yes	No	Yes	No

As nouns, mental capacity and brain are synonyms defined as:

Other synonyms of mental capacity include brainpower, learning ability, mentality, wit.

mental capacity (noun)	brain (noun)
mental ability	that part of the central nervous system that includes all the higher nervous centers; enclosed within the skull; continuous with the spinal cord
	mental ability
	that which is responsible for one's thoughts, feelings, and conscious brain functions; the seat of the faculty of reason
	the brain of certain animals used as meat
	someone who has exceptional intellectual ability and originality

mental capacity (verb)	brain (verb)
	hit on the head
	kill by smashing someone's skull