In a dilute acetic acid solution, water is ignored for sample equilibrium. The concentration of water is shown to be 55.5 M. Since the concentration changes very little in a dilute solution, the concentration of water can be considered fairly constant. Number of moles was calculated by the volume of the vessel the water was in.
First do the back calculation from mol/L to g/L. The molecular mass of water is. Since the mass of the water would be divided by the molecular mass to get the moles, the moles can be multiplied by the molecular mass to get back to mass of the water. Therefore,
This is indicates that the mass of water is equivalent to1000 grams per liter. So we have 1000 mL of volume and 1000 g of mass.
Hence, a one liter flask does which determine the concentration of water as 55.5 M.