[jhndms]

When  NaCl is added(below solubility limits) to water it breaks down into ions and solvated by the solute molecules. My questions are:

1) NaCl has a pKa of 6.7-7.3, which means at least some of the substance is not being split into ions then how can we say that it has been dissolved?

2) If some process(say a plant) removes Na+ and Cl- ions in disproportionate quantities then the un-dissociated NaCl will split into Na+ and Cl- to match the equilibrium dissociation constant. Am I right to say that?

Thank You.

 

[renge ishyo]

There is not necessarily a relationship between pK_a and degree of dissociation if the species involved don't directly contain a proton (which in this case neither one does). For example, the degree of dissociation of NaCl is best described by looking at the K_sp for NaCl which is 36 (source: http://dl.clackamas.edu/ch105-04/solubili.htm). This means that dissociation into sodium and chloride ions is favorable in general. The pK_a (which is very close to neutral) probably refers to the fact that the solvated sodium ion can participate in some acid-base chemistry by having one of the protons on it's complexed water molecules removed, thus making it a very weak acid. Adjusting the relative amounts of Cl^- and Na⁺ in solution can probably explain the small shift in pK_a values, but the given range is still pretty close to neutral.