Chemistry / Chem 1239 · Procedure · 60–90 seconds
Solving Problems with the Ideal Gas Law
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Solving an ideal-gas-law problem means identifying which three of pressure, volume, moles, and temperature are given, converting each to the units matching the chosen gas constant, then rearranging PV = nRT to isolate and solve for the unknown fourth quantity.
A tank holds 655 g of methane at 25 °C and 745 torr. Converting mass to moles using methane's molar mass, 16.04 g/mol, gives about 40.8 mol. Converting temperature to kelvin (298 K) and pressure to atmospheres (0.980 atm), then rearranging PV = nRT to V = nRT / P, gives a volume of about 1020 L.
Plugging in a Celsius temperature, or a pressure in torr while using the atmosphere-based gas constant, produces an answer wrong by a large, easily missed factor, often an order of magnitude off rather than a small rounding error.
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