help please

jason1988

hey guys i cant seem to work out how to do this question any help would be greatly appreciated! :smile:

Graham’s law states that the rate of effusion of a gas is inversely proportional
to the square root of the molecular mass.
An unknown gas consisting of carbon, hydrogen and oxygen (CxHyO) diffuses
out of a pinhole 3.873 times slower than helium (He). Use Graham’s law to
find the molecular mass of the unknown gas and hence deduce its molecular
formula.
[Ar(H)]

Reply 1

melcarac

It means that sqrt(M CxHyO)/ sqrt (M He) = 3.873. You know the molecular mass of helium, so calculate the molecular mass of the unknown compound

Reply 2

jason1988

Ok so i just work out what divides by 2 (sqrt of he) to give 3.8 but how do i find out what molecules are in the unknown substance?

Reply 3

melcarac

You get the molecular mass of the unknown substance. There is one oxygen atom in its molecule. Substract the atomic mass of oxygen, and then substract the atomic mass of carbon as many times as possible (or just divide it by twelve and take the integer as the number of carbon atoms). And what's left is the number of hydrogen atoms.