How much water must be added when 125 mL of a 2.00 M solution of HCl is diluted to a final concentration of 0.400M?

To determine the amount of water to be added, we use the equation:

M1V1 = M2V2

where M1 and M2 are the concentrations and V1 and V2 are volumes. We calculate as follows:

2.00 (125) = 0.4 (V2)

V2 = 625 mL

Water to be added = 625 - 125 = 500 mL of water

Hope this answers the question.