ppm is mg/L so 1.08e3ppm is 1.083e3mg/L. you can convert this to mole/L by simply converting the mg to grams then to mole using m/Mr where m is the mass in grams and Mr is the molar mass of Na (periodic table).
once you get that, you will end up with a mol/L. Then you need to work out the volume of water using its density. Then with that volume, you determine how much Na will be present in that volume of water. this is possible because, you know how much Na in mole are there in one Liter of the solvent from the molarity you first calculated.
Sea water contains an average of 1.08 x 10^3 ppm of Na+ ion. Calculate the molar concentration of Na+ given that the average density of sea water is 1.02 g/ml.
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