seawater contains approximately 3.5% NaCl by mass and has a density of 1.02 g/mL. What volume of seawater contains 1.0 g of sodium?

Answer 1

I would do this.
3.5% NaCl by mass means
3.5g NaCl/100 g solution. The density of th solution is 1.02 g/mL; therefore, the 100 g soln has a volume of mass/density = 100g/1.02 = 98.04 mL.
Next, I would convert 3.5 g NaCl to g Na. That is easily done by
3.5g NaCl x (molar mass Na/molar mass NaCl) = 1.4 g Na. This is an estimate; you can do it more accurately. In reality, then, our solution is 1.4 g Na/98.04 mL.
(1.4 g Na/98.04 mL) x ? mL = 1.0 g Na.
Solve for ?mL.