When the solution boils, only the solvent is converted to gas. The solute remains in the solution, so its amount does not change but the concentration does. The amount of solute is:
Amount of solute = 80 g/L * 7 L = 560 g
In order for concentration to be 200 g/L
200 g/L = 560 g/Volume
Volume = 2.3 L
There should be 2.3 L of brine solution. So, the amount of water boiled off is:
Amount of Water = 7 L - 2.3 L = 4.2 L water
