DALTON'S LAW OF PARTIAL PRESSURES (p. 391 392 in text book)


1.)  The sum of the pressures of the individual gases equals the total pressure exerted by the mixture of gases.

PTOTAL = Pgas1 + Pgas2 + ...

EXAMPLE:  A mixture of oxygen and nitrogen exerts 1.1 atm of pressure.  What is oxygen's partial pressure if the pressure of the nitrogen gas is 0.8 atm?

2.)  The pressure of a gas "collected over water" is equal to the atmospheric pressure minus the vapor pressure of the water.

Pgas = Patm - Pwater

EXAMPLE:  A 44.6 mL sample of carbon dioxide is collected over water at 765 mm Hg pressure and 25 oC.  What is the vapor pressure of the "dry" gas?  (The vapor pressure of water at 25 oC is 23.76 mm Hg.  You will have to look up this value on a table or this value must be given to you.)



3.)  The partial pressure of a gas is equal to the "mole fraction" multiplied by the total pressure.

Px =       (moles x)      .  Ptotal                Px = partial pressure of certain gas
           (total moles)

* "mole fraction" =      (moles x)  
                              (total moles)

EXAMPLE:  A mixture of gases contains 2.0 moles of He and 4.0 moles of oxygen.  If the mixture exerts a pressure of 801 torr, what is the partial pressure of the oxygen?


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