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:
P total
= P
Gas 1
+ P
Gas 2
+ P
Gas 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 and P
N2
= 0.285 torr
= 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 + P oxygen gas
P oxygen
= 760 torr - 0.285torr - 593.525 torr
P oxygen
= 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.

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.
P
H2O
P total
= 760 torr
= 23.8 torr therefore
760 torr = P oxygen
P oxygen
+ 23.8 torr
= 736 torr