Question

Which of the compounds, NaF or MgO, would be soluble in water? justify your answer.

Answer 1

A priori, we would assume sodium fluoride to be the more water soluble species....why?

Explanation:

The dissolution of an ionic salt involves the breaking of STRONG chemical bonds: consider the reaction of sodium chloride with water...

`NaCl(s) stackrel(H_2O)rarrNa^(+) +Cl^-`..

...sodium and chloride ions are present in aqueous solution as their aquated ions. This also occurs for the reaction of sodium fluoride with water....

`NaF(s) stackrel(H_2O)rarrNa^(+) +F^-`..

On the other hand, reaction of `MgO` involves the breakage of stronger electrostatic bonds between a dication and a dianion...

`MgO(s) stackrel(H_2O)rarrMg^(2+) +2HO^-`..

And for this reason, we would anticipate the reaction of the magnesium oxide to be LESS favourable. In terms of entropy, the magnesium dication would also be disfavoured....and thus both entropy and enthalpy disfavour the dissolution of the oxide salt...

So much for theory. As chemists, as physical scientists, we should illustrate our answer with some data. This site quotes an aqueous solubility of `40*g*L^-1` at `20` `""^@C` with respect to sodium fluoride...

And magnesium hydroxide has `K_"sp"=5.61xx10^-12`...the which would give rise to lower solubility, in the `mg*L^-1` range...