What is plasma osmolarity? Plasma osmolality?

Conc. electrolytes. These refer to the concentration of electrolytes in the body/plasma, or the electrolyte-water balance. These measure the same thing, only that osmolality is the number of osmoles per kilogram of solvent or osm/kg, while osmolarity is the number of osmoles per liter of solution or osm/ltr. Osmolality excludes the solute, while osmolarity includes it, so osmolality has a slightly higher reading.
Minor difference. Osmolarity is number of moles in a liter of solution and osmolality is number of moles of solute in a kg of water. Normal plasma osmolality is about 290 mosmol/kg.