DALTON’S LAW OF PARTIAL PRESSURE

Recall that the total pressure of a mixture of gases is equal to the sum

of the partial pressures of the component gases. Dalton’s Law of

Partial Pressure can be stated mathematically as:

Ptotal = PGas 1 + P Gas 2 + PGas 3 + …

Three of the primary components of air are carbon dioxide, nitrogen, and oxygen. In a sample containing a

mixture of only these gases at exactly one atmosphere pressure, the partial

pressures of carbon dioxide and nitrogen are given as P CO2 = 0.285 torr

and PN2 = 593.525 torr. What is the partial pressure of oxygen?

SOLUTION: Did you remember to change the one atmosphere

pressure to torr?

1 atm is STANDARD PRESSURE, which is 760 torr.

760 torr = 0.285 torr + 593.525 torr + Poxygen gas

Poxygen = 760 torr - 0.285torr - 593.525 torr

Poxygen = 166.190 torr

Gases produced in the laboratory are often collected over water. Gas collected over water is always mixed with water vapor,

which exerts a pressure of its own known as water vapor pressure. Since water

vapor pressure of varies with temperature, you must look up the value of P H2O vapor at the temperature of the experiment in a

standard reference table like that on your reference sheet and like the one provided

on your lab table. Use Dalton’s Law of Partial Pressures to calculate the following.

Use Dalton’s Law of Partial Pressures to calculate the

following.A 250 ml sample of oxygen is collected over water at 25⁰ C and 760 torr pressure. What

is the pressure of the dry gas alone?

P total = P O2 gas + P H2O vapor

SOLUTION: From the water vapor pressure table reference the vapor pressure of water at 25⁰C is 23.8 torr.

Ptotal = 760 torr P H2O = 23.8 torr therefore

760 torr = Poxygen + 23.8 torr Poxygen = 736 torr