Question

Calculate the activity coefficients of 1 mm Na+ and 1 mm Cl– in acetonitrile at 25°C, assuming the density of acetonitrile is 0.7857 g/mL, and its dielectric constant εR is 37.5. The density of water at 25°C is 0.9970 g/mL, and its dielectric constant εR is 78.57. Compare the activity coefficients to those for water at the same concentration and temperature, and explain any difference.

Answer 1

An activity coefficient is a factor used in thermodynamics to account for deviations from ideal behaviour in a mixture of chemical substance

a mean activity coefficient of the dissolved electrolyte, , must be used. For a 1:1 electrolyte, such as NaCl it is defined as follows:

where γ₊ and γ_- are the activity coefficients of the cation and anion respectively. This definition tacitly assumes a degree of 100% ionic disociation of the electrolyte.