We are given the dioxin which is present in 2.6 liters of this water. Also there is a density of 1.00 g/ml. You can apply the density formula wherein density is equal to mass of the substance divided by the volume displaced.
D = M/V
1 gram / liter = M / 2.6 liters
M = 2.6 grams
So there are 2.6 grams of dioxin in the sample.
