national capital vs Nouakchott

national capital and Nouakchott both are nouns.

Word	Noun	Adjective	Verb	Adverb
national capital	Yes	No	No	No
Nouakchott	Yes	No	No	No

national capital (noun)	Nouakchott (noun)
the capital city of a nation	capital of Mauritania; located in western Mauritania near the Atlantic coast