AAMC 11 PS #35 wrong choices

[Tenebrus]

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Hey guys,

Just a quick queston on two of the wrong choices from this question:

35. A jar is half-filled with water, the temperature is equilibrated to 25ºC, and the jar is sealed. The pressure inside the jar will decrease if which of the following occurs?

A. The jar is colled to 15ºC
B. The jar is heated to 35ºC
C. The water vaporizes inside the jar
D. Air escapes from the water

I realize all you need to see to answer this is that A is correct based on PV=nRT, but I'm curious as to what would happen if C or D were true.

Would C/D increase pressure, or keep it constant? I'm inclined to think it would remain constant, but I'm not sure at all... any insight?

 

[sciencebooks]

Hey guys,

Just a quick queston on two of the wrong choices from this question:

35. A jar is half-filled with water, the temperature is equilibrated to 25ºC, and the jar is sealed. The pressure inside the jar will decrease if which of the following occurs?

A. The jar is colled to 15ºC
B. The jar is heated to 35ºC
C. The water vaporizes inside the jar
D. Air escapes from the water

I realize all you need to see to answer this is that A is correct based on PV=nRT, but I'm curious as to what would happen if C or D were true.

Would C/D increase pressure, or keep it constant? I'm inclined to think it would remain constant, but I'm not sure at all... any insight?

For C, you'd have more in the gas phase moving around with a higher velcoity, bumping, and causing pressure.

D appears to be the same situation. A gas forming from liquid.

 

[Tenebrus]

Guess that makes sense... thanks!

 

[MangoPlant]

For C, you'd have more in the gas phase moving around with a higher velcoity, bumping, and causing pressure.

D appears to be the same situation. A gas forming from liquid.

but for C wouldn't the vaporization also lower temperature? I guess the effect of a liquid turning to gas > the temperature/pressure reduction caused by vaporization.

 

[richkaj]

In order for something to vaporize it goes from liquid to a gas. This needs kinetic energy sufficient to do so. Temperature is directly related to kinetic energy. If the temperature increases the pressure increases as well due to more collisions in the sealed container.