[need this right now, how do i do
a 300 ml sample of gas was collected at stp weighs .90g. what is the molar mass of this gas?

If your collecting at STP, you can use the converting constant of 22.4 litters/ One Mole. So, you would have convert ml to L and then go from there.

Also, when working with grams, you usually need to know the gas your working with. As one mole equals the atomic mass of an element in grams.