Question

Calculate the ionic strength of (a) a coastal seawater of salinity 17.5 (i.e., one-half of typical open ocean salinity) in mol kg-1 and (b) 0.7 mol/l NaCl and (c) another solution of 0.1 mol/l MgCl2

Answer 1

ionic strength = 1/2Sum of Ci^2Zi^2
multply the concentration mole/kg of each ion at S =35 by 0.5 to get the concentration at 17.5
then find the charge squared of the major ions Na+1, K+1,Mg+2 , Ca+2, Sr+2,Cl-, So42-
HCO3-,Br-,F- and Co3-2

ionic strength =1/2[ z² Na⁺¹ *CNa⁺¹ +z² Cl^-1* CCl^-1]

                  = 1/2 [1² *0.7 + 1²*0.7]

                 = 0.7M

ionic strength   = 1/2[z² Mg⁺² *C Mg⁺² +2* z² Cl^-1]

                     = 1/2 [2² *0.1 +2*1² *0.1]

                   = 1/2 [4*0.1 +2*0.1]

                  = 1/2*0.6 = 0.3M