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)# #"SO"_4^(2-)#
#b)# #"HCO"_3^(-)#
#c)# #"CO"_3^(2-)#
#d)# #"NO"_3^(-)#
1 Answer
Likely, sulfate... but you would have to do the experiment yourself to verify this.
...And this reaction is given by...
#"Na"_2"SO"_4(aq) + "MgCl"_2(aq) -> "NaCl"(aq) + "MgSO"_4(s)#
#s_(MgSO_4) ~~ "35.1 g/100 mL water"# at#25^@ "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
#"MgSO"_4(aq)# shifts the equilibrium between crystallization and [dissolution + solvation], towards crystallization, because its dissolution process is exothermic.#DeltaH ~~ -"91.2 kJ/mol"# .
As for the others...
#s_(Mg(HCO_3)_2) = "0.077 g/100 mL water"# at#25^@ "C"#
#s_(MgCO_3) = "0.0139 g/100 mL water"# at#25^@ "C"#
#s_(Mg(NO_3)_2) = "large"# at#25^@ "C"#
And the first two clearly precipitate at room temperature and pressure. Where on Wikipedia did I get this data?
