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Please could you tell me if I am going in the right direction.
I am trying to write molecular formula for As2O3

I make molecular mass 197.8
gas density 17.8 to 3 sig figs

2.0 x density of gas at STP / density of hydrogen

17.8/0.0899 = 396 to 3 figs

How do I go about matching it to molecular mass? I can see that 396/2=198 but if that's the right way I need to understand how I arrived at that point? I would be very grateful for some advise.

I am confused by the reference to H2 and the 2.0 x value. Was that an experiment you performed.
Is that 17.8 g/L (no units mentiioned), if so, then multiply by 22.4 L/mol = 398.7 for the molar mass. Check my thinking. Or have I missed something?

Relative molecular mass of gas =

2.0 X (density of gas) at STP/ density of hydrogen gas at STP

Im trying to determine molcular formula
to match it to the empiricalformula in its gaseous state

You need to multiply the density of the gas by the molecular mass of oxygen which is 3 x 16 x 17.8 then divide by density of oxygen at STP which should show you that the empirical formula is nearly the same as the molecular formula i.e. as2o3

Well this is wrong but serves you right S103 student for cheating

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