This conversion would be based on the assumption that your gas mixture behaves in an ideal manner. 1 mole of an ideal gas has a volume of 24.2 liters at standard conditions. A SCOTTY 17 cylinder contains 17 liters or 0.7 moles [17L X (1 mole/ 24.2L)] of gas at a pressure of 260 psig. The total weight of your gas mixture can be calculated using this information and the molecular weights of all minor components in the mixture. As you did not provide the composition of your gas mixture, I will base an example on a 100ppm n-Hexane, balance Air gas mixture. Percent Concentration of n-Hexane = 100ppm = 0.010% = 0.00010 Percent Concentration of Oxygen = (1,000,000ppm - 100ppm) X 20.9% = 20.898% = 0.20898 Percent Concentration of Nitrogen = (1,000,000ppm - 100ppm) X 79.1% = 79.092% = 0.79092 Total moles of gas = 0.7 Moles of n-Hexane = (0.7 moles X 0.00010) = 0.00007 moles Moles of Oxygen = (0.7 moles X 0.20898) = 0.146286 moles Moles of Nitrogen = (0.7 moles X 0.79092) = 0.553644 moles Weight of n-Hexane = 86.18 g/mole X 0.00007 moles = 0.0060312 grams Weight of Oxygen = 32.00 g/mole X 0.146286 moles = 4.6811520 grams Weight of Nitrogen = 28.01 g/mole X 0.553644 moles = 15.5075684 grams Total weight of gas = 0.0060312 + 4.6811520 + 15.5075684 = 20.1947516 grams = 20.2 grams