Mrs. is correct
It"s Mrs. Ms (Miss) is for single women.
Mrs. (from Mistress) is the usual title for a married woman; Ms. doesn't specify whether or not she's married.
(And I'm not going to mention the doctorates/other gender-neutral titles or that you didn't specify "some woman," but that's just from hanging with gay friends...)
Mrs. is the formal term as it is derived from Mister's or Mr's.
But Ms. is just as respectful as Mrs. as some people would preferred not to be objectified. However if you don't know if someone is married or not its safe to call someone Ms.