Investigate what the components of the gas are.
Measure the total pressure of that gas.
Calculate the mole fraction for each component of the gas. It's the number of moles of a certain gas divided by the total number of moles for the entire gas.
Multiply the mole fraction of the first component by the total pressure.
Repeat the previous step for all components.
You can verify the results by adding all partial pressures together and check if they sum up to the total pressure.
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