Question

what will happen to the human rbc if kept in 1.5% of salt solution

Answer 1

there is a concentration gradient
so along this gradient, cytoplasm will flow out to solution and the cell will shrink.

Answer 2

It depends on what kind of salt solution you are using.

If the salt solution is hypotonic with respect to RBCs (that is salt solution is rarer in osmotically active particles than intracellular plasma) then the water molecules will osmosis into the RBCs from the solution and it might lead to rupturing of RBCs.

If the salt solution is hypertonic (opposite of hypotonicity) then the water molecules will osmosis out of the RBCs leading to plasmolysis (extensive shrinkage of RBCs due to hypertonic environment).

However there may also exist Isotonicity between the intracellular and extracellular fluids (that is concentration of osmotically active particles is same in both fluids). Such Isotonicity leads to no net movement of water molecules between the fluids. This Isotonicity is arrived at 0.3M(molar) NaCl solution (approx).

PS: only water molecules shuttle across the RBC membrane as it is selectively permeable to certain particles, and solutes present extracellularly here are not one of them.