Question

A sample of MgCO3 is dissolved in dilute HCl and the solution is neutralized with ammonia and buffered with NH4Cl / NH4OH. Disodium hydrogen phosphate reagent is added to the resulting solution. A white precipitate is formed. What is the formula of the precipitate?

• A. Mg3(PO4)2
• B. Mg(NH4)PO4
• C. MgHPO4
• D. Mg2P2O2

Answer 1

The Correct Option is B

When magnesium carbonate ($MgCO_3$) is dissolved in dilute hydrochloric acid (HCl), it reacts to form magnesium chloride (MgCl₂) and carbonic acid (H₂CO₃):

$MgCO_3 + 2HCl \rightarrow MgCl_2 + H_2CO_3$ The carbonic acid then decomposes into water and carbon dioxide: 

$H_2CO_3 \rightarrow H_2O + CO_2$ After this reaction, magnesium chloride (MgCl₂) remains in solution. 

Next, the solution is neutralized with ammonia (NH₃) and buffered with NH₄Cl/NH₄OH. This creates a slightly alkaline medium. 

When disodium hydrogen phosphate (Na₂HPO₄) is added to the solution, it reacts with magnesium ions (Mg²⁺) to form a precipitate. The reaction leads to the formation of magnesium ammonium phosphate (Mg(NH₄)PO₄), which is a white precipitate. 

The formula of the precipitate is: 

Answer: Mg(NH₄)PO₄