CO2 at 1 atm pressure (about 1 bar) and 0°C (32°F) has a density of 1.9771 g/l. At room temp (20°C) the density will be ~ 1.9771 g/L * 273.15/293.15 = 1.8422 g/L. At 1 bar over pressure (2 bar absolute pressure) the density will be double, or ~3.7 g/L. (The 3.9 from @Vale71 omitted the temperature correction.) A "5" gal keg actually has a volume closer to 20 L. So, if you set your CO2 regulator to 14.7 psi (1 bar over pressure), you will need 3.7 g/L * 20 L = 74 g of CO2 to push all the liquid out of a completely full keg.
Edit: If you set your regulator to 5 psi (a more reasonable pressure for pushing StarSan out of a keg) you would use 1.8422 g/L * 20 L * (14.7 + 5)/14.7 = 49.4 g of CO2.
Brew on
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