Osmolality of a molecule in solution is a function of the number of separate particles. One molecule of glucose is a single particle, but one molecule of sodium chloride splits into two separate particles (ions), one of Na+ and one of Cl-. And 0.15 x 2 = 0.3. So, in your example, although the molar concentration of glucose is twice that of NaCl, the osmolality is the same. If you had 0.3 M NaCl, it's osmolality would be 0.3 x 2 = 0.6 oM.