Seawater has a density of 1.05 g/mL and contains 3.10% sodium chloride by mass. What volume of seawater, in liters, are needed to provide 2.50 kg of sodium chloride?

So 100 g sea water contains 3.10 grams NaCl. You want 2500 g NaCl; therefore, 100 g sea water x 2500/3.10 = # grams sea water needed to furnish that 2500 g NaCl. Then mass sea water = volume sea water x density sea water. You know mass from above, you know the density from the problem, solve for volume sea water. Post your work if you get stuck.