Not understanding the logic of this GC question (DAT Destroyer)

[Gunacaik]

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Hey everyone,

This question is from the 2016 edition of the DAT Destroyer, #251 in the General Chemistry section. It reads:

A 500 ml gas sample collected over H2O at 740mmHg and 25 C (celsius) is studied for analysis. What is the volume of the dry gas at STP if the vapor pressure of H2O at 25 C is 24mmHg?

(The choices are given in the equation form, solving for V)

I understand that we would use the equation: P1V2 / n1T1 = P2V2 / n2T2 (moles are the same in this case)
But why would we subtract 740 - 24 to get the vapor pressure of water?

 

[DAT Destroyer]

Hey everyone,

This question is from the 2016 edition of the DAT Destroyer, #251 in the General Chemistry section. It reads:

A 500 ml gas sample collected over H2O at 740mmHg and 25 C (celsius) is studied for analysis. What is the volume of the dry gas at STP if the vapor pressure of H2O at 25 C is 24mmHg?

(The choices are given in the equation form, solving for V)

I understand that we would use the equation: P1V2 / n1T1 = P2V2 / n2T2 (moles are the same in this case)
But why would we subtract 740 - 24 to get the vapor pressure of water?

Great question. You need to subtract because it contains water !!! You must use the DRY GAS !!!!! Think of it as a wet shirt. Before you hang it out to dry, you squeeze the water out. Whenever you collect a gas over water, the pressure of the water needs to be subtracted . This ensures that you are not factoring in any outside pressures other than the gas in question.

Hope this helps.

Dr. Romano

 

[Gunacaik]

Great question. You need to subtract because it contains water !!! You must use the DRY GAS !!!!! Think of it as a wet shirt. Before you hang it out to dry, you squeeze the water out. Whenever you collect a gas over water, the pressure of the water needs to be subtracted . This ensures that you are not factoring in any outside pressures other than the gas in question.

Hope this helps.

Dr. Romano

Haha! Thank you Dr. Romano. Makes much more sense now.

 

[DAT Destroyer]

Haha! Thank you Dr. Romano. Makes much more sense now.

Keep up the good work, glad to see you are working hard during this holiday season!