Assume you have 1 L of seawater.

**Step 1. Calculate the mass of the seawater**

#"Mass of seawater" = 1000 color(red)(cancel(color(black)("mL seawater"))) × "1.025 g seawater"/(1 color(red)(cancel(color(black)("mL seawater")))) = "1025 g seawater"#

**Step 2. Calculate the mass of #"NaCl"#**

#"Mass of NaCl" = 1025 color(red)(cancel(color(black)("g seawater"))) × "3.50 g NaCl"/(100 color(red)(cancel(color(black)("g seawater")))) = "35.88 g NaCl"#

**Step 3. Calculate the moles of #"NaCl"#**

#"Moles of NaCl" = 35.88 color(red)(cancel(color(black)("g NaCl"))) × "1 mol NaCl"/(58.44 color(red)(cancel(color(black)("g NaCl")))) = "0.6139 mol NaCl"#

**Step 4. Calculate the molarity of #"NaCl"#**

#" Molarity" = "0.6139 mol"/"1 L" = "0.614 mol/L"#