Ideal gas expansion/compression

In summary: In this specific case, the volume decreases while the pressure increases, but the number of moles remains the same. As mentioned, it is important to have more information in order to accurately solve this type of problem.
  • #1
format1998
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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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  • #2
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!
 
  • #3
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.
 

1. How do ideal gases expand and compress?

Ideal gases expand and compress due to changes in temperature and pressure. When heated, the gas molecules gain kinetic energy and move farther apart, causing the gas to expand. On the other hand, when the gas is cooled, the molecules lose kinetic energy and move closer together, resulting in a decrease in volume or compression.

2. What is the ideal gas law and how is it used to describe gas expansion and compression?

The ideal gas law is PV = nRT, where P is pressure, V is volume, n is the number of moles, R is the gas constant, and T is temperature. This law is used to describe the relationship between these variables in an ideal gas, which expands or compresses according to this equation.

3. What is the difference between isothermal and adiabatic expansion/compression?

In isothermal expansion/compression, the temperature remains constant, while in adiabatic expansion/compression, there is no heat exchange with the surroundings. This means that in isothermal processes, the gas behaves in a more predictable manner, while in adiabatic processes, the temperature and pressure change more significantly.

4. How does the volume of an ideal gas change during expansion/compression?

According to Boyle's Law, the volume of an ideal gas is inversely proportional to the pressure, assuming constant temperature. This means that as pressure increases, the volume decreases during compression, and vice versa during expansion.

5. What factors affect the expansion/compression of an ideal gas?

The expansion and compression of an ideal gas are affected by temperature, pressure, and volume. In addition, the type of process (isothermal or adiabatic) and the number of moles of gas also influence the expansion/compression behavior. External factors, such as the presence of a piston or the addition/removal of heat, can also impact the expansion/compression of an ideal gas.

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