Mixed gas refers to the gas containing two or more effective components, or the content of non effective components exceeds the specified limit. The mixture composed of several gases is the working medium commonly used in engineering. Mixed gases are usually studied as ideal gases.

The nature of the gas mixture depends on the type and composition of the gas. The composition of mixed gas can be expressed in three ways.

① Volume composition: the ratio of the partial volume of the constituent gas to the total volume of the mixed gas, expressed in ri. The so-called partial volume refers to the volume of the constituent gas under the temperature and total pressure of the mixed gas.

② Mass composition: the ratio of the mass of the constituent gas to the total mass of the mixed gas, expressed in wi.

③ Molar composition: The mole is the unit of measure of a substance. If the number of basic units (atoms, molecules, ions, electrons or other particles) contained in a system is equal to the number of 0.012 kg carbon-12 atoms, the amount of substance in the system is 1 mole. The ratio of the number of moles of the constituent gas to the total number of moles of the mixed gas, expressed in xi.

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