to determine the formula of a new substance, one of the first steps is to find the?

Question

1. molar mass
2. percent composition
3. volume at STP
4. number of particles per mole

Answer 1

1] Would be a good start, but is nowhere near the whole story
2] Also a good start
3] Volume @ STP is only meaningful if it is in terms of mass, in which case it is the same information as 1] above.
4] Irrelevant - tells you NOTHING

Answer 2

There are two right answers, and two that don't tell you anything specific to the compound.