I think hydrogen is approximately half as heavy as helium. Hydrogen has only one proton; helium has two. I can’t remember about how many neutrons there are in each.
Hydrogen is explosive, rather than inflammable. A subtle difference, but important in some circumstances.
To calculate the volume of air/helium in a tyre:
You need to think of the innter tube as a cyclinder.
Calculate the cross sectional area. That is Pi x (radius squared)
That’s the radius of the cross section of the tube. So if the inflated tube is, for example, 3 inches across, the radius is 1.5 inches and you square that and get 2.25.
Pi is 3.14, so you multiply 3.14 X 2.25 = 7.065 inches.
Now you need to calculate the length of the cyclinder. That will be very roughly the circumference of the wheel. In fact it will be a bit less than the circumference of the rim of the tyre, and a bit more than the circumference of the rim. What you could do is measure the diameter of the rim, measure the diameter of the inflated tyre, and average the result.
e.g. Rim 26 inches, tyre 28 inches, average 27 inches.
The circumference is Pi x diameter
3.14 x 27 inches = 84.78 inches
Now the voulme of the tube is the lenght of the cylinder, multiplied by the cross sectional area:
84.78 x 7.065 = 599 cubic inches.
But to find how much air/helium the tube contains, you need to multiply the volume by the pressure in Bars/Atmospheres.
1 Bar is approximately 14 pounds per square inch. So if the tube is inflated to 56 psi, that’s 4 Bars, so the tube contains the equivalent of 599 x 4 cubic inches of air/hydrogen. Say 2,400 cubic inches.
Now all you need to do is find the weight of air/helium per cubic inch.
Remember to use the same units all the way through: either inches or centimetres.
Personally, I shall carry on using air and not worrying.