Question

Circle all of the following solutions that are isotonic with red blood cells (0.30 osmol) ?

0.30 M NaCl 0.30M C₆H₁₂O₆ 0.10 M NaCL 0.10 M Na₂CO3

Answer 1

Correct answers: 0.30 M C₆H₁₂O₆

                            0.10 M Na₂CO₃

Osmolarity = n x m

where n = number of particles into which the molecules dissociates. For glucose (C₆H₁₂O₆), n = 1. For NaCl, n=2. For Na₂CO₃, n=3.

m = molarity of the solution

Therefore, for 0.30 M glucose

osmolarity = 1 x 0.30 = 0.30 osmol

For 0.30 M NaCl, osmolarity = 2 x 0.30 = 0.60 osmol

For 0.10 M Na₂CO₃, osmolarity = 3 x 0.10 = 0.30 osmol

For 0.10 M NaCl, osmolarity = 2 x 0.10 = 0.20 osmol

Isotonic means having same osmotic pressure. Therefore 0.30 M glucose and 0.10 M Na₂CO₃ are isotonic with red blood cells (0.30 osmol)