Question

1. A) Which calculated activity coefficient for Ca²⁺is greater -- that in pure water or that in NaCl solution? Explain why the activity coefficients differ in this way.

B) Why is gypsum (or any ionic solid) more soluble in the presence of an inert (non-interacting) salt such as NaCl?

C) Why do we typically 'ignore' activity coefficients when calculating equilibrium constants (or even when calculating pH)? Is this a justifiable practice, why or why not?

Answer 1

1.

A:

activity is essentially the interaction of ions in solution

in pure water, we have low activity

in water with plenty of ions, there is higher activity per each species

B:

they are more soluble since there is no other ions interacting, therefore the water-salt interactions are higher, that is, the equilibirum shifts toward ionic solution

C:

Typically, they are small, or cancel each other, or are not to be considered since we use pretty dilute solutions. IT is ok depending on the applciation, i.e. for learning and in the lab its ok; for forensics, analytical applcations, pharmaceutical applications, you should not ignore it