Ionic Strength Formula
Measure total ion concentration in solution
Formula
I = 1/2 Σ ci zi²
- I = ionic strength (mol/L)
- ci = molar concentration of ion i
- zi = charge of ion i
- Σ = sum over all ions
Example: 0.1 M NaCl
Naâº: c = 0.1 M, z = +1
Clâ»: c = 0.1 M, z = -1
I = 1/2 [0.1(1)² + 0.1(-1)²]
= 1/2 [0.1 + 0.1] = 0.1 M
Answer: I = 0.1 M
Example: 0.1 M CaClâ‚‚
Ca²âº: c = 0.1 M, z = +2
Clâ»: c = 0.2 M (two per formula unit), z = -1
I = 1/2 [0.1(2)² + 0.2(-1)²]
= 1/2 [0.4 + 0.2] = 0.3 M
Answer: I = 0.3 M (note: higher than molarity due to charge)
Importance
- Used to calculate activity coefficients via Debye-Hückel equation
- Affects solubility, reaction rates, and equilibria
- Higher charge ions contribute more to ionic strength (z² factor)
- For 1:1 electrolyte, I = molarity; for others, I > molarity