Daltons Law for Gases

In 1808, John Dalton stated that the total pressure in a system of gases is the same as the sum of the pressures of its components. This statement, made in his book "A New System of Chemical Philosophy", is now known as Dalton's Law, and amounts to saying that the total pressure is the sum of the pressures which each gas would exert if it were confined alone in the volume occupied by the mixture.

For example, we know that at 90C a mol of water vapor occupies approximately 30 litres at atmospheric pressure. A mol of ethanol vapor at the same temperature would occupy the same volume at that temperature, so if both mols were to be combined at atmospheric pressure then the total volume would be 60 litres. But if we were now able to remove the ethanol vapor from the mix, but keep the same volume (the volume of the mixture), then the mol of water would exert only half an atmosphere pressure, and vice versa. In this case, the "partial pressures" of both the water and the ethanol are each half an atmosphere. You can arrive at the same conclusion another way by using the ideal gas equation, PV =nRT. If you double the volume of any vapor at a given temperature you halve the pressure.

Expressed in molecular terms used for vapors, Y1 = mols of substance 1 in vapor/ (mols of substance Y2 = mols of substance 2 in vapor/ (mols of substance Ptotal = total pressure of the system P1 = Y1Ptotal = vapor partial pressure of substance 1 P2 = Y2Ptotal = vapor partial pressure of substance 2

In other words, from the molecular point of view, the ONLY important factor is the relative number of molecules present. The difference between the vapor pressure of a substance on its own, (PN*), and its partial vapor pressure in a mixture (XNPN* for liquids, or YNPTOTAL for vapors) is very important.

in the mix in the mix

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  • david
    How is dalton's law used in fractional distillation?
    2 years ago

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