Question

Find the ionic strength of a 4 mM of CaCl2. What are the activities of Ca2+ and Cl-.

Answer 1

Since CaCl₂ produces one Ca²⁺ ion and two Cl^- ion, hence the molarities of the ions in this case would be as follows:

[CaCl₂] = 4 mM =0.004 M

m₊ = 0.004 , m_- = 2 x 0.004 = 0.008;

z₊ = 2; z_- = 1

Hence, Ionic strength, I = 1/2( 0.004 x 2² + 0.008 x 1²) = 0.012.

The mean activity coefficient is given by

  

"A" value, temperature and solvent not mentioned in question.

For water at 25 ^oC, A = 0.509

-0.509 X 2 x 1 x (0.012) ^1/2

   = -0.111

So, 0.774