how to prepare 1M,50ml buffer solution by taking Na2HPO4 and NaH2PO4.

Answer 1

In biochemistry the molarity of the buffer is the sum of both the acid and the conjugate base. Your post just gives the total but not the breakdown; therefore, I would suggest 50 mL(0.05L) for 0.5M each. That is 0.5 x 0.05 = 0.025 moles.
g each = moles x molar mass.