What is the formula for rubidium arsenide?

Answer 1

A little more information is needed. Rhenium is a transition metal, and as such, can have multiple valence states. If an ionic charge is known, for example, +2, then it can be assumed that the formula for the oxide would be RhO. If percent composition by mass is known, for example, 86.5% rhenium by mass, then math can be done to determine the formula of the oxide, which in this case, would also be RhO.

Answer 2

It depends since rhenium is a multivalent metal. If it's charge is +4 then, Re3As4. If +7 then, Re3As7.