Calculate the molar concentration?

Question

sea water contains average of 1.08 * 10^-3 ppm of Na+.

calculate the molar concentrations of Na+ given that the average density of sea water is 1.02 g/ml.

Answer 1

On average, seawater has a salinity of about 3.5%, or 35 parts per thousand. This means that every 1 kg of seawater has approximately 35 grams of dissolved salts (mostly, but not entirely, the ions of sodium chloride: Na+, Cl-). The average density of seawater at the surface of the ocean is 1.025 g/ml;

These figures do not fit at all with your data on the Na+ concentration of sea water. Using the above, 35,000 ppm NaCl in sea water , which based in NaCl would give the concentration of Na+ as:

23/58.5*35000 = 13761ppm or 1.38*10^4 ppm You say 1.08*10^-3 or 0.00108ppm which is obviously not correct.

I will use my figures:

1.38*10^4 * 1.02 = 1.41*10^4mg of Na+ per litre solution

Atomic mass Na = 22.990 = 2.3*10^4 mg

Molarity = 1.41*10^4/2.3*10^4 = 0.612M as Na+