Which one of the following is the correct molecular formula of ammonium carbonate if the valency of ammonium ion is (+1) and carbonate anion is (-2) ?
- A.(NH₄)₂CO₃
- B.NH₄(CO₃)₂
- C.(NH₃)₂CO₃
- D.NH₄CO₃
Correct answer
Explanation
The correct answer is A, (NH₄)₂CO₃. Two ammonium ions, each with a charge of +1, are needed to balance one carbonate ion with a charge of -2.
To write the formula of an ionic compound, the valencies of the two ions are crossed over and the smallest whole-number ratio is kept. Ammonium (NH₄⁺) has valency 1 and carbonate (CO₃²⁻) has valency 2, so the compound needs 2 NH₄ for every 1 CO₃. A polyatomic ion that appears more than once is placed in brackets with the number outside, giving (NH₄)₂CO₃. Ammonium carbonate is a white solid that gives off ammonia, and it was once used in smelling salts and as a leavening agent in baking.
B is wrong: it has two carbonate ions for one ammonium ion, giving a charge of -3, not zero.
C is wrong: NH₃ is neutral ammonia, not the ammonium ion.
D is wrong: one NH₄⁺ cannot balance one CO₃²⁻; the charges add up to -1.
Exam tip: cross over the valencies and use brackets for a repeated polyatomic ion, as in (NH₄)₂SO₄ and Ca(OH)₂.
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