Locate the pressure valve on the transport tank and take note of the pressure being exerted by the gas within. It is often given in pounds per square inch (PSI) and must be converted to atmospheres. The conversion factor for PSI to atmospheres is 1 atmosphere for every 14.7 PSI. For example, if a tank reads as 120 PSI, it will posses 120/14.7 = 8.16 atmospheres of hydrogen.
Take the temperature in the room where the hydrogen tank resides. If the hydrogen tank is outside, simply take the outside temperature. Convert any temperature taken in Celsius to degrees Kelvin. Kelvin equals the degrees in Celsius plus 273. Temperatures in Fahrenheit need to be converted to Celsius and then to Kelvin. Convert Fahrenheit to Celsius by subtracting 32 from the degrees Fahrenheit then divide by 1.8. (See Tips.)
Multiply the pressure, in atmospheres, by the volume of the tank containing the hydrogen. The volume of the tank is indicated on the tank's label. For example, if you have a 20-liter tank: 20 liters x 8.16 atm. = 163.27.
Multiply the temperature, in degrees Kelvin, by the gas constant. The gas constant is equal to 8.314472. A tank at 300 degrees Kelvin times the gas constant, commonly symbolized as a capital letter "R," yields 300 * 8.314472 = 2,494.34.
Divide the number ascertained in Step 3 by the number ascertained in Step 4. The result is the amount of hydrogen in moles. A "mole" is a standard measure of atoms and molecules used commonly in chemistry and physics. Traditionally, to find the amount of substance in grams, you multiply the number of moles of a substance by a conversion factor of moles per gram. However, hydrogen has a conversion factor of 1; therefore, the mole value and its number of grams are identical: 2,494.34 x 163.27 = 1,357.50 moles = 1,357.50 grams of hydrogen.