* Density of water: Water has a density of approximately 1 gram per milliliter (g/mL). This means 1 milliliter of water weighs 1 gram.

* Conversion: Since we have 400 mL of water, and each mL weighs 1 gram, we have: 400 mL * 1 g/mL = 400 grams.

Therefore, there are 400 grams of water in 400 ml.