If you have a 2 M stock solution of Tris Buffer, a 0.5 M stock solution of Magnesium chloride, and a bottle of crystalline sodium chloride. Use these reagents to make 500 ml of a solution containing 50 mM Tris, 5 mM of MgCl2, and 0.25 M sodium chloride.
the volume of the final solution V = 500 mL = 0.5 liters.
In the solution, there are c1 = 50 mM = 0.05 M Tris,
c2 = 5 mM = 0.005 M of MgCl2,
c3 = 0.25 M NaCl.
the moles of NaCl is n3 = V * c3 = 0.5 * 0.25 = 0.125 mol.
The molecular mass of NaCl is 58.5 g/mol
therefore, the mass of NaCl is
m = 0.125 * ...
It shows how to make the desired solution from the stock. The solution is detailed and has a '5/5' rating.