It often happens that a gas mixture is created at standard conditions (STP), in which ideal-gas law is applicable, and then pressurized and added in storage cylinders. The goal is to calculate the properties of the gas mixture at storage pressure, but due to the high pressure and density, ideal-gas law cannot be used. Molar concentration is used in real-gas equations to calculate the properties of gas mixtures. Mole fractions don't change after compression and can be used to find the properties of the mixtures at storage conditions.
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