Question

Calculate the concentration in osmolarity of a solution prepared by adding 5.00 g NaCl (molar mass = 58.44) to a 100 mL volumetric flask, filling the flask to the mark with distilled water, and mixing.

Answer 1

Osmolarity = i x Molarity         i = Van't Hoff factor

   i = Van't Hoff factor for NaCl = 2

Hence,

For NaCl , Osmolarity = 2 x Molarity  

Molarity of NaCl = ( mass / molar mass) x (1/volume of solution in Litres)

                           = ( 5.00 g / 58.44 g/mol) x (1/0.1 L)

                            = 0.855 M

Molarity of NaCl = 0.855 M

Osmolarity = 2 x Molarity  

                   = 2 x 0.855 M

                    = 1.71 osmol/L