Question

What is the difference between carbon dioxide, and silicon dioxide? How do we formulate the solubility product for the salt `A_2B_3`

Answer 1

1. Carbon dioxide is a molecular species; silicon dioxide is non-molecular.
2. `K_(sp) = [A]^2[B]^3`

Explanation:

As you know, carbon dioxide and silicon dioxide are isoelectronic. They are not isostructural. The shortness of the (first-row) `C-O` bonds allows effective overlap between the p -orbitals of `C` and `O`, in addition to the strong `sigma` bond formed. The result is a `C=O` double bond. On the other hand, the corresponding p -orbitals on second row `Si` are too diffuse to allow effective `pi` bonding between `Si` and `O`, with the result that `O` bridges to another silicon to form an infinite array of `Si-O-Si-O` bridges that have no molecular boundary.

As to your `A_2B_3` salt, do you mean a salt of the form such as calcium phosphate, i.e. `Ca_3(PO_4)_2`? Such a salt would be particularly insoluble (as are most phosphates).

We would write the equation for its dissolution in water for as:

`Ca_3(PO_4)_2(s) rarr 3Ca^(2+)(aq) + 2PO_4^(3-)(aq)`

The solubility constant is as standard, the concentrations of the ions raised to the power of their stoichiometric coefficients:

`K_(sp) = [PO_4^(3-)]^2[Ca^(2+)]^3`

Alternatively, consider the solubility of ferric sulfate:

`Fe_2(SO_4)_3(s) rarr 2Fe^(3+) + 3SO_4^(2-)`

`K_(sp) = [Fe^(3+)]^2[SO_4^(2-)]^3`

This should not be too water soluble, but check this.

If this is not want you wanted I apologize.