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

17.8g/l
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

To determine the molecular formula for As2O3, you need to consider the molar mass and the empirical formula of the compound.

The molar mass of As2O3 can be calculated by adding the atomic masses of the elements in the empirical formula. Arsenic (As) has an atomic mass of approximately 74.92 g/mol, while oxygen (O) has an atomic mass of approximately 16.00 g/mol. So, the molar mass of As2O3 can be calculated as:

2 * molar mass of As + 3 * molar mass of O
= 2 * 74.92 g/mol + 3 * 16.00 g/mol
= 149.84 g/mol + 48.00 g/mol
= 197.84 g/mol (rounded to three significant figures)

Now let's address the confusion related to the reference to H2 and the 2.0 x value. It seems like you are trying to calculate the molar mass of the gas, where the density of the gas is compared to the density of hydrogen gas at STP (standard temperature and pressure). However, this approach is not necessary for determining the molecular formula of As2O3.

To match the molar mass calculated above (197.84 g/mol) to the empirical formula of As2O3, you should divide it by the empirical formula mass. The empirical formula mass can be calculated by adding the atomic masses of the elements in the empirical formula. For As2O3, the empirical formula mass is:

2 * atomic mass of As + 3 * atomic mass of O
= 2 * 74.92 g/mol + 3 * 16.00 g/mol
= 149.84 g/mol + 48.00 g/mol
= 197.84 g/mol (same as the molar mass)

Since the molecular mass (197.84 g/mol) and the empirical formula mass (197.84 g/mol) are the same, it indicates that the empirical formula of As2O3 is also the molecular formula.

Therefore, the molecular formula for As2O3 is As2O3 itself.

Note: It's important to understand the principles behind these calculations rather than relying on cheating or dishonest practices. Ethics and academic integrity are crucial for personal growth and learning.