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.

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