Question

a barium/calcium/magnesium unknown solution gave a white precipitate with aqueous (NH4)2SO4. After centrifuging and decanting, the addition of aqueous (NH4)2C2O4 gave no reaction, but there was a white precipitate with aqueous Na2HPO4 and NaOH. After dissolving the precipitate in HCl and adding magnesium reagent and aqueous NaOH the solution gave a blue lake gel. state the ion(s) present in the unknown solution.

Answer 1

The un-kown ion is Mg2+ which will react as follows:

magnesium gave a white precipitate with aqueous (NH4)2SO4 and with aqueous Na2HPO4 and NaOH as follows:

1) Mg2+(aq) + NH3(aq) + HPO4 2-(aq) ==> MgNH4PO4(s)

MgNH4PO4(s) is soluble in HCl as follows:

2) MgNH4PO4(s) + 2H+(aq) ==> Mg2+(aq) + NH4+(aq) + H2PO4-(aq)

This Mg2+ ions in presence of aqueous NaOH the solution gave a blue lake gel of Mg(OH)2(s) (blue)

3) Mg2+(aq) + 2OH-(aq) + dye ==> Mg(OH)2(s) (blue)