If a red blood cell is placed in a 0.5 salt solution what will occur

The red blood cell will shrink.

In salt solution, the water potential is a lot lower than the cell cytoplasm. In that way, water molecules will flow from the inside of the cell to the salt solution by osmosis. The red blood cell will loose so much water that it shrinks.

But we don't have to worry about that because this won't occur inside our body, since there is mechanism in our body that regulates the concentration of blood.