:eek! :
a sample is made of 18g of C and 81.5g of H... the molecular mass of the compound is 104g/mole

calc. the empirical formula...
calc. the molecular formula

OK.. so I got the moles, I think.:confused: .
C= 18g X 1mole = 1.4987g
1 12.01( Atomic mass)

H= 1.5g X 1mole = 1.5g
1 1g ( Atomic mass)

now I don't know what is next.. so I did..
1.4987g/mole / 1.5g/mole = .99g( I don't know what this number is for)

and I have no idea what to do next...

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Double check your units!

Now, I did not check the math but the values seem reasonable (I am assuming the 81.5g H that you listed is a typo and should be 1.5g based on your calculations), however, you are converting to moles so you need to have moles as your units.

18 g C = 1.4987 mol C

1.5 g H = 1.5 mol H

Now we need to look at lowest whole number ratios of the elements present.

When you divided you were establishing the ratios.

Both of them equal out to 1.

So we have a 1:1 ratio of C to H

Therefore our empirical formula is CH.

Since the molar mass of the molecular formula is 104g

We can divide this out using the mass of the empirical formula.

104 ÷ 13.02 which will give us about 8.

Now factor that over the empirical formula to get

C8H8 as the molecular formula