Ideal gas expansion/compression

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When an ideal gas expands or compresses, the number of moles typically remains constant unless gas is added or removed. Changes in volume and pressure can occur without altering the moles of gas. In the given example, a decrease in volume by a factor of 0.5 and an increase in pressure by a factor of 4.5 do not imply a change in the number of moles. To solve problems involving ideal gases, at least three of the four variables (pressure, volume, number of moles, temperature) must be known. Understanding the conditions of the system is crucial, as ideal gas behavior can vary based on whether the process is isobaric, isothermal, or isochoric.
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this may be a stupid question but i have to ask as my book doesn't specifically say, it'll help me figure out a problem in my homework. So, here it goes...

When an ideal gas expands or is compressed, does the number of moles change or does it remain the same? if it remains the same, does that mean only the volume changes. For instance, in the case of compression of an ideal monatomic gas. Gas is compressed and the volume decreases by a factor of .5, pressure is increased by a factor of 4.5. Does the # of moles increase, decrease, or neither?

Thank you in advance. Any and all help is much appreciated!
 
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In order to solve these problems, you need data about any three of the four variables (P, V, n, T)

Your example only specifies two (P, V). Without a third specification, it's impossible to know if one or both mass and temperature have changed.

I'd like to say "typically" ideal gas problems don't have mass flows associated with them; but that's not really true. There are plenty of open-system ideal gas problems. So proceed with the assumption that mass is constant at your own peril.

You really do need more info. Ideal gases can be compressed and expanded isobarically (const. P), isothermally (const. T), isochorically (const. volume), or in a closed system (const. mass). The problem really should state it--read carefully--or it's incomplete and not well posed. Only mathematicians can solve a two-unknowns problem with one equation. ;-)

format1998 said:
this may be a stupid question but i have to ask as my book doesn't specifically say, it'll help me figure out a problem in my homework. So, here it goes...

When an ideal gas expands or is compressed, does the number of moles change or does it remain the same? if it remains the same, does that mean only the volume changes. For instance, in the case of compression of an ideal monatomic gas. Gas is compressed and the volume decreases by a factor of .5, pressure is increased by a factor of 4.5. Does the # of moles increase, decrease, or neither?

Thank you in advance. Any and all help is much appreciated!
 
If not specified otherwise, assume the number of moles of gas does not change. For the moles to change, either some more gas has to be added to the initial gas present, or some of the initial gas escapes.

When the number of moles is constant, the other quantities (P, V, or T) can still change.
 
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