Do moles change with volume?

Question

Does increasing or decreasing volume change the moles in a solution, or does that just affect the concentration?

 

If I have 0.035711 moles in 100mL and I'm asked how many moles in 5mL of this solution, wouldn't it be the same?

Answer 1

You are asking 2 different questions, and they describe totally different situations.

 

First question:  If there is no net flow of solute into or out of the volume during change in volume of the solvent, the number of moles of solute remains the same.  However, the concentration changes inversely with volume.

 

Second question:

 

This refers to the same solution, but a smaller volume of it.  The concentration is the same, so the number of moles changes directly with volume.  Answer this by setting up a proportion:

 

(0.035711 moles)/(100 mL) = x/(5 mL)

 

x = (5 mL)(0.035711 moles)/(100 mL) = ? moles