What volume of 1.0 M sodium phosphate, to the nearest tenth of a liter, must be used to make 4.0 L of 0.80 M sodium phosphate?

To solve this we use the equation,

M1V1 = M2V2

where M1 is the concentration of the stock solution, V1 is the volume of the stock solution, M2 is the concentration of the new solution and V2 is its volume.

1 M x V1 = 0.80 M x 4 L

V1 = 3.2 L