vapor pressure question

[alanan84]

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A 500mL gas sample is collected over water at a pressure of 740mmHg and 25C. What is the volume of the dry gas at STP? (STP = 1atm and 0C) Given: Vapor pressure of H2O at 25C = 24mmHg.

I know how this is set up except for the correct answer has you subtract 24mmHg from 740mmHg. Why? I initially thought it should be added...

 

[greg81988]

Which formula did you use to solve this problem and how is it set up?

 

[greg81988]

Is the correct answer 430 mL?

 

[nze82]

A 500mL gas sample is collected over water at a pressure of 740mmHg and 25C. What is the volume of the dry gas at STP? (STP = 1atm and 0C) Given: Vapor pressure of H2O at 25C = 24mmHg.

I know how this is set up except for the correct answer has you subtract 24mmHg from 740mmHg. Why? I initially thought it should be added...

Notice that the question specifically tells you that gas is collected over water, but their looking for the volume of the DRY GAS (Not the mixture of gas and water). 740mmHg is the total pressure, which equals to Pressure of the Gas + Pressure of the water. Now, to figure out the initial pressure of the DRY GAS, we must deduct the water pressure from the total pressure (740-24mmHg). This is your P1. Then simply use the following formula to solve for V2:

P1V1/T1 = P2V2/T2

So:

V2 = (P1V1T2)/(T1P2)