Seawater contains approximately 3.5% by mass and has a density of 1.02 g/mL. What volume of seawater contains 5.5g of sodium.
What a badly worded question! There is no way you can calulate the mass of sodium from the information given as you need to know what proportion of the 3.5% by mass are sodium salts.
What you could do is assume that all of the 3.5% is sodium chloride. So 100 g of seawater contains
3.5 g of NaCl or
3.5 g x23/58.4 of sodium.
=1.38 g sodium
so to get 5.5 g of sodium we need
5.5 g x 100/1.38 of sea water
To find the volume
volume = 398.6 g/1.02 g ml^-1
But check the maths!