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!