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How to calculate partial pressure?

A sample of oxygen is collected over water at a total pressure of 690.7 mmHg at 19 °C. The vapor pressure of water is 16.5 mmHg at at that temperature. The partial pressure of O2 is?
a)0.9305 atm
b)0.9349 atm
c)0.9088 atm
d)
 

Dalton's law of partial pressure states that the total pressure in a mixture of gases is equal to the sum of the partial pressures.

In the case where a gas is collected over water, the measured pressure of the gas is a due to the fact that both the collected gas (O2 in this gas) and water vapor are both present.  Gases collected in this manner are referred to as "wet gases".

So, applying Dalton's Law gives us:

                Pwet gas =   P O2    +    P H2O

Since we are given that the measured pressure of the mixture (P wet gas) is 690.7 mm Hg and that the water vapor pressure is 16.5 mm Hg, the equation becomes

              690.7 mm Hg  =   P O2   +   16.5 mm Hg

Solve for P O2

Thank you very much, it really helped!!!