Question

The solubility constant of silver bromate is 5.5 x 10-5, whereas the solubility constant of calcium sulfate is 2.4 x 10-5, while they are close to the same solubility due to their exponents being the same, calcium sulfate has a smaller leading number and so it is less soluble than silver bromate. What is more soluble, magnesium hydroxide or magnesium carbonate?

Answer 1

Magnesium hydroxide is a very weak base while magnesium carbonate is the salt of a weak base - magnesium hydroxide and carbonic acid. Salts of the latter kind tend to stay in equilibrium with their base and acid froms by abstracting protons or hydroxide ions from the medium. In this case, magnesium hydroxide is a weaker base than carbonic acid and so, magnesium tends to abstracts hydroxide ions better than the rate carbonate abstracts protons from the water it is dissolved in, to produce hydroxide ions. This makes the overall medium also basic. Thus, it can be observed that both are basic compounds.

Among these, taking into account the high instability of magnesium cations and carbonate anions in their pristine state in water and comparing that with the higher stability of their conjugate acid and base respectively, we can roughly state that the carbonate salt tends to dissociate better in water, to form conjugates to stabilize themselves. It is well known that ionic solids dissolve in water by dissociating into their respective ions. Thus, we can say that magnesium carbonate has a higher solubility than magnesium hydroxide, which agrees with the extremely low Ksp of magnesium hydroxide (5.6x10^-12) and the relatively higher solubility product of magnesium carbonate (1x10^-8).