Yes, IMPUREST is a Scrabble word!

Letter values in Scrabble US: I 1, M 3, P 3, U 1, R 1, E 1, S 1, T 1.

Is it a valid word?

IMPUREST is a valid word in the official Scrabble dictionary (US) and the UK Scrabble Dictionary, but not in Words with Friends (ENABLE).

combined with extraneous elements
(used of persons or behaviors) immoral or obscene
having a physical or moral blemish so as to make impure according to dietary or ceremonial laws