For 10 points, name this measure of how much of a given substance exists within a given volume, most often measured in molarity, or moles per liter.
Ilkovic solved a partial differential equation involving derivatives of this quantity for a semi-infinite space.  Another PDE involving this quantity was transposed to Lagrangian coordinates by Taylor and Aris and says that the time derivative of this quantity is proportional to its second spatial derivative.  The molar flux is equal in magnitude to the molecular diffusivity times the gradient of this according to Fick's Law, and it can be measured in normality, molality, or percent by volume or weight.  For 10 points, name this measure of how much of a given substance exists within a given volume, most often measured in molarity, or moles per liter.
Ilkovic solved a partial differential equation involving derivatives of this quantity for a semi-infinite space.  
{concentration}