I recently calculated for concentration needed to my solution to get -1.5MPa PEG 6000 and I got 0.05g PEG/g H2O but I don't know how to convert this to moles/ L which is more familiar to me and more easier for preparing solutions. Please help.Thank you very much.

0.05 g PEG/molar mass = mols. Easy enough to do.

mass solution = density x volume.
You know mass (1 gram), use density and solve for volume of 1 gram of the solution. The problem is that you don't know the volume of the solution. Convert volume to liters, then M = mols/L and you have it.