Question 7.12: Calculate the ionic strength of a simple 1:1 electrolyte, su......

Inserting values into Equation (7.31) we obtain

I=\frac{1}{2}\sum_{i=1}^{i=i}c_{i}z_{i}^{2}             (7.31)

We next insert concentration terms, noting that one sodium ion and one chloride are formed per formula unit of sodium chloride (which is why we call it a 1:1 electrolyte). Accordingly, the concentrations of the two ions, \left[\mathrm{N}\mathrm{a}^{+}\right]{\mathrm{~and~}}\left[\mathrm{Cl}^{-}\right], are the same as [NaCl], so

I={\frac{1}{2}}\{([\mathrm{NaCl}]\times1)+([\mathrm{NaCl}]\times1)\}

so we obtain the result for a 1:1 electrolyte that I_{\mathrm{(NaCl)}}=\mathrm{[NaCl]}.
Note that I has the same units as concentration: inserting the NaCl concentration [NaCl] = 0.01 mol \mathrm{dm}^{-3}, we obtain I = 0.01 mol \mathrm{dm}^{-3}.