Question

Sea water contains an average of 1.08*10^3ppm of Na+ and 270ppm of SO4^2-. A. Calculate the molar concentration of Na+ and SO4^2- given that the average density of water is 1.02g/ml.

B. The pNa and pSO4 for sea water.

Answer 1

Solution :-

Ppm means 1 mg per kg

Lets assume we have 1 kg solution then volume of the solution is

(1 kg * 1 L / 1.02 kg) = 0.98039 L

Now lets calculate the mass of the Na+

1.08*10^3 ppm means 1080 mg of the Na+

Now lets calculate the moles of Na+

(1080 mg * 1 g / 1000 mg)*(1 mol / 23 g) = 0.04696 moles of Na+

Molarity of the Na+ = 0.04696 mol / 0.98039 L

                                      = 0.0479 M

Now lets calculate the mass of SO4^2-

270 ppm means 270 mg SO4^2-

Lets calculate moles of SO4^2-

(270 mg * 1 g / 1000 mg)*(1mol/ 96.06 g) = 0.002811 mol

Molarity of SO4^2- = 0.002811 mol / 0.98039 L

                                    = 0.002867 M

Part B

pNa+ = -log [Na+]

          = -log [0.0479]

           = 1.32

pSO4^2- = -log [SO4^2-]

                = -log[0.002867]

                = 2.54