Question

(6) A seawater sample has the following composition: Mass Concentrations Mol. Weight Molar Concentrations (a) Convert these mass concentrations to molar units (moles per liter) assuming density = 1.026 kg/L. Express the results using the appropriate M, mM, MuM, nM etc. notation (for example, 1.35x10^-3 M would be 1.35 mM, 1.35x10^-2 would be 13.5 mM and 1.35x10^-4 would be 135 MuM). (b) What is the salinity of this sample?

Answer 1

(a) density = mass/volume

volume = mass/density

molarity = moles/L

For the given data,

                                 Mass concentration              mol. wt. (g/mol)             molar conc. (moles/L)

sodium                          10,700 ppm                        39.0983                            266.7341 mM

potassium                         399 ppm                          40.078                               9.703 mM

calcium                             412 ppm                           24.305                              16.522 mM

magnesium                      1294 ppm                         137.327                               9.184 mM

barium                                 25 ppb                           35.5                              0.686 micromolar

chloride                          19,350 ppm                         36                                  523.879 mM

sulfate                             2.700 ppm                         96.065                            27.394 mM

bicarbonate                       145 ppm                            61                                 2.317 mM

silicate(Si(OH)4)                   8 ppm                        96.0855                         81.15 micromolar

(b) Salinity of solution

= sum of salts dissolved

= 35,008.025 ppm

In terms of molarity (moles/L) of salt calculated = 855.815 mM