Final answer:
A 0.15 M NaCl solution is hypotonic to red blood cells because it is less concentrated than the isotonic standard of 0.16 M NaCl, resulting in a potential influx of water into the cells.
Step-by-step explanation:
To determine whether a 0.15 M NaCl solution is hypotonic, isotonic, or hypertonic, we must compare it to the osmolarity of normal red blood cell cytoplasm. A red blood cell will maintain normal volume and shape in an isotonic solution, will swell in a hypotonic solution, and will shrivel in a hypertonic solution. A typical isotonic NaCl solution for red blood cells has an osmolarity of 0.32 osmol/L. Since each NaCl unit produces two ions, the isotonic concentration for NaCl would be 0.32 osmol/L divided by 2, which equals 0.16 M. Therefore, a 0.16 M NaCl solution is isotonic. Given that the question references a 0.15 M NaCl solution, it is slightly less concentrated than the isotonic standard of 0.16 M, which indicates that the solution is hypotonic to red blood cells.