Final answer:
Red blood cells are isotonic to a 0.9% sodium chloride solution, but hypertonic to a 0.9% sodium sulfate solution.
Step-by-step explanation:
Red blood cells are isotonic to a solution that is 0.9% sodium chloride (NaCl) because the osmolarity or concentration of solutes inside the cells matches the osmolarity of the NaCl solution. In an isotonic solution, there is no net movement of water into or out of the cell, so the red blood cells maintain their normal shape and volume.
However, red blood cells would be hypertonic to a solution that is 0.9% sodium sulfate (Na2SO4). This is because sodium sulfate dissociates into three ions (Na+, Na+, and SO4^2-) when it dissolves, whereas sodium chloride only dissociates into two ions (Na+ and Cl-) when it dissolves. The higher concentration of ions in the sodium sulfate solution causes more water to flow out of the red blood cells, leading to cell collapse and shrinkage.
In summary, red blood cells are isotonic to a 0.9% sodium chloride solution but hypertonic to a 0.9% sodium sulfate solution.