Question

When disodium hydrogenphosphate solution is added to a solution containing $M{g}^{2+}$ ions, the colour of the precipitate formed is:

• A. Black
• B. White
• C. Red
• D. Yellow

Answer 1

The correct option is B White

Test for $M{g}^{2+}$ Ions
If group V is absent the solution may contain magnesium carbonate, which is soluble in water in the presence of ammonium salts because the equilibrium is shifted towards the right hand side.
The concentration of carbonate ions required to produce a precipitate is not attained. When disodium hydrogenphosphate solution is added and the inner walls of the test tube are scratched with a glass rod, a white crystalline precipitate of magnesium ammonium phosphate is formed which indicates the presence of $M{g}^{2+}$ ions.
$M{g}^{2+}+N{a}_{2}HP{O}_4\to \underset{\text{Magnesium ammonium phosphate}\text{(White precipitate)}}{Mg\left(N{H}_4\right)P{O}_4}+N{H}_{4}OH+2N{a}^{+}+H_{2}O$