Why Do Water Softeners Prefer Calcium Over Sodium Ions?

[minime191]

Homework Statement

Explain the following features of ion-exchange resins in a home water softener.
(a) Negatively charged sulfonate “sites” on resin molecules attract Ca2+
(aq) ions better than Na+ (aq) ions. (Consider the ion charges in your answer.)

(b) Sodium ions from saturated NaCl(aq) replace calcium ions during regeneration. (Consider the concentration and collision theory in your answer.)

Homework Equations

More concentration results in higher collision.

The Attempt at a Solution

I think I've got a figured out;
The negative charge attracts the positive charges. But since calcium has 2, it has a higher charge hence is attracts better than Na.
I don't know what to do about b. Can I please get some direction?

 

[Borek]

This is an equilibrium process.

 

[Querious]

part b talks about how the ion exchange resin could be 'recharged' with sodiums ions required in the process of softening water as mentioned in part a.

Try this http://en.wikipedia.org/wiki/Ion_exchange_resin" page for a short and concise description of the process.

The key word to tackling question b is saturated NaCl(aq).