pressure

[chiddler]

---

Members do not see this ad.

i've been staring at this for some 30 minutes

need halp

What is the pressure in atmospheres of a column of gas in a closed tube above mercury if the height difference at sea level between a connected column of mercury open to the atmosphere and the closed column above mercury is 317 mm?

I find the wording very confusing and have trouble visualizing what is going on.

Thanks!

 

[MedPR]

i've been staring at this for some 30 minutes

need halp

What is the pressure in atmospheres of a column of gas in a closed tube above mercury if the height difference at sea level between a connected column of mercury open to the atmosphere and the closed column above mercury is 317 mm?

I find the wording very confusing and have trouble visualizing what is going on.

Thanks!

The wording definitely is confusing. Where is this from?

 

[aSagacious]

1 atmosphere (atm) is a unit of pressure (as is torr, bar, psi, and mmHg).

1 atm = 760 mm Hg

If the difference between the column heights is 317 mmHg, then...

317mmHg
--------------- = 0.417 atm
760 mmHg

Conceptually, mmHg is simply a historical unit referencing the use of mercury manometers (see below).

How it worked is you partially fill a U-shaped tube with mercury (of known density, temperature, etc) with one side closed and the other exposed to some pressure (usually of the atmosphere). The higher the pressure on the open end of the tube, the farther the fluid is forced into the tube, thus resulting in the apparent height difference of the tubes. Imagine putting your mouth over the open end of the tube and alternately sucking and blowing, what would happen the height of the liquid?

 

[chiddler]

it's berkeley review

IMG]http://www.sensorsmag.com/files/sensor/nodes/2001/1073/fig5.gif[/IMG]

How it worked is you partially fill a U-shaped tube with mercury (of known density, temperature, etc) with one side closed and the other exposed to some pressure (usually of the atmosphere). The higher the pressure on the open end of the tube, the farther the fluid is forced into the tube, thus resulting in the apparent height difference of the tubes. Imagine putting your mouth over the open end of the tube and alternately sucking and blowing, what would happen the height of the liquid?

i don't understand how you got those numbers

 

[SLC]

aSagacious gave a very concise and clear explanation, if you know that 760mmHg is equal to 1 atm, then it becomes a simple fraction problem Take your difference in height (as recorded in mmHg) and divide by 760 to give you pressure difference in atm.

My only hesitation is that your explanation of the question doesn't include whether the column height is 317 mm above or below the column that's exposed to the atmosphere. The absolute value will be identical, but you can't say whether the pressure difference is +0.417atm, or -0.417atm without that information.

 

[chiddler]

okie dokie thanks very much for the help

 

[SLC]

i don't understand how you got those numbers

Oh and those numbers used stem from the fact that the question mentions the apparatus being set up at sea level. 1 atmosphere (atm.), or 760mmHg is the air pressure as recorded at sea level.

So we can say that the pressure of the column in the sealed end is either +0.417atm, or -0.417atm depending on the positioning of the menisci in the manometer. However; if you weren't told that the apparatus was placed at sea level, you could still state the difference in pressure, rather than the pressure itself, which in this case, would be either +0.417atm, or -0.417atm from the outside pressure wherever the apparatus was set up.

The 1atm at sea level, and 1 atm-760mmHg is something you'll need to have memorized, and be familiar with for many pressure questions on the MCAT.

Does that help, or does that make things more confusing?

 

[chiddler]

Oh and those numbers stem from the fact that the question mentions the apparatus being set up at sea level. 1 atmosphere (atm.), or 760mmHg is the air pressure as recorded at sea level.

The 1atm at sea level, and 1 atm-760mmHg is something you'll need to have memorized, and be familiar with for many pressure questions on the MCAT.

sorry i meant i wanted a slightly more detailed explanation. i know those numbers.

but it's ok i've spent an unfortunately excessive amount of time with this and i think i understand it.

thanks again.