# Which of the following anions would precipitate the most in the reaction of sodium salts with magnesium chloride? What could be done to increase the solubility of the precipitate?

## a) ${\text{SO}}_{4}^{2 -}$ b) ${\text{HCO}}_{3}^{-}$ c) ${\text{CO}}_{3}^{2 -}$ d) ${\text{NO}}_{3}^{-}$

Aug 9, 2017

Likely, sulfate... but you would have to do the experiment yourself to verify this.

...And this reaction is given by...

${\text{Na"_2"SO"_4(aq) + "MgCl"_2(aq) -> "NaCl"(aq) + "MgSO}}_{4} \left(s\right)$

${s}_{M g S {O}_{4}} \approx \text{35.1 g/100 mL water}$ at ${25}^{\circ} \text{C}$

which is quite high. But contrary to common expectation, by boiling, the solubility decreases.

Magnesium sulfate is one of the only examples I can think of that decreases in solubility at higher temperatures. You can read more about this here, but in short...

Increasing the temperature for ${\text{MgSO}}_{4} \left(a q\right)$ shifts the equilibrium between crystallization and [dissolution + solvation], towards crystallization, because its dissolution process is exothermic. $\Delta H \approx - \text{91.2 kJ/mol}$.

As for the others...

${s}_{M g {\left(H C {O}_{3}\right)}_{2}} = \text{0.077 g/100 mL water}$ at ${25}^{\circ} \text{C}$

${s}_{M g C {O}_{3}} = \text{0.0139 g/100 mL water}$ at ${25}^{\circ} \text{C}$

${s}_{M g {\left(N {O}_{3}\right)}_{2}} = \text{large}$ at ${25}^{\circ} \text{C}$

And the first two clearly precipitate at room temperature and pressure. Where on Wikipedia did I get this data?