A sample of propane gas contains `9.36xx10^24*"hydrogen atoms"`. What mass with respect to carbon, and hydrogen, AND propane, does this quantity represent?

Now if I specify a mole of propane I specify `6.022xx10^23` individual propane molecules. Why should we use such an absurdly large number? Well, because it is a fact that `6.022xx10^23` individual carbon atoms have a mass of `12*g` (or near enuff), and `6.022xx10^23` individual hydrogen atoms have a mass of `1*g`. These molar masses are listed on the Periodic Table, and there should be a copy beside you now.

Now it follows that a mole of propane SPECIFIES `3xx6.022xx10^23` carbon atoms, and `8xx6.022xx10^23` hydrogen atoms, and so we multiply thru by the molar masses to give a molar mass for the propane molecule as....

`3xx12.011*g*mol^-1+8xx1.00794*g*mol^-1=44.10*g*mol^-1`

That is the background. It is specified that there are `9.36xx10^24*"hydrogen atoms"`, and thus represents a molar quantity of ..................

`(9.36xx10^24*"hydrogen atoms")/(6.022xx10^23*"hydrogen atoms"*mol^-1)=15.54*mol`

And clearly, we can divide this by 8 to give the molar quantity of `1.94*mol` WITH RESPECT TO PROPANE. Do you agree?

And so (finally), there are.....

`1.94*molxx3*"carbon atoms"xx6.022xx10^23*mol^-1`

`=3.51xx10^24*"carbon atoms"`.

And as to the mass of the sample, I take the product...

`"Number of moles"xx"molar mass of propane"`

`=1.94*molxx44.1*g*mol^-1~=90*g`, i.e. I get an answer in grams as required.