How do you calculate the concentration of substance X in mM if given volume of substance X and total volume in mL? Is there a need to incorporate the molar mass of substance X?

Question

How do you calculate the concentration of substance X in mM if given volume of substance X  and total volume in mL? Is there a need to incorporate the molar mass of substance X?

aaronq · Answer

To find the concentration in millimoles/L (which is 1/1000 of a mole in 1 L), you need to find the moles of the substance. In order to do that you need to find the mass, but when you're only given volume you need to know the density.

TL;DR You need the density because you need to find the mass/moles of the solute first.