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
Answer
Why
Correct — A, (NH₄)₂CO₃. The ammonium ion NH₄⁺ has a valency of +1 and the carbonate ion CO₃²⁻ has a valency of −2. For a neutral compound the total positive and negative charges must balance, so two NH₄⁺ ions (giving +2) are needed for one CO₃²⁻ ion (−2). Enclosing the polyatomic ammonium group in brackets with subscript 2 gives (NH₄)₂CO₃.
Why the others are wrong
- (b)NH₄(CO₃)₂ — This puts one +1 ammonium with two −2 carbonates, a net charge of −3, which is not neutral; the subscripts are on the wrong ions.
- (c)(NH₃)₂CO₃ — This uses NH₃ (ammonia, a neutral molecule) instead of the ammonium ion NH₄⁺, so it is not the correct formula of ammonium carbonate.
- (d)NH₄CO₃ — One +1 ammonium with one −2 carbonate leaves a net charge of −1, so the compound is not neutral; two ammonium ions are needed.
Concept
The formula of an ionic compound is written by balancing the charges (valencies) of its ions so the total is zero. In the criss-cross method, the numerical valency of each ion becomes the subscript of the other, and any polyatomic ion taken more than once is enclosed in brackets.
Here NH₄⁺ (valency 1) and CO₃²⁻ (valency 2) must balance, so two ammonium ions pair with one carbonate ion. Recognising ammonium as NH₄⁺ (not NH₃) and remembering the brackets rule points straight to (NH₄)₂CO₃.
Key facts
- The ammonium ion NH₄⁺ has valency +1; the carbonate ion CO₃²⁻ has valency −2.
- In a neutral compound the total positive charge equals the total negative charge.
- Two NH₄⁺ balance one CO₃²⁻, giving (NH₄)₂CO₃.
- A polyatomic ion taken more than once is written inside brackets before its subscript.
Two ammonium ions balance one carbonate ion — (NH₄)₂CO₃, option (a).
Study next
Common traps
- Forgetting the brackets around a polyatomic group taken more than once.
- Confusing ammonia (NH₃) with the ammonium ion (NH₄⁺).
Balance the charges of the two given ions — two +1 ammonium ions for one −2 carbonate ion — and write the polyatomic group in brackets.
Related PYQs
Which of the following anions has a valency of -3?
- (a) Nitride
- (b) Nitrate
- (c) Sulphide
- (d) Sulphate
Answer(a) Nitride
Same concept — the valency of ions, which decides how a formula is written. That NDA item identifies an anion of valency −3; this one balances the +1 and −2 valencies of ammonium and carbonate to build a formula.
Which of the following sets of elements has the same valency?
- (a) Na, Mg, Ca
- (b) Na, Mg, Al
- (c) Mg, Ca, K
- (d) Mg, Ca, Ba
Answer(d) Mg, Ca, Ba
Same idea — valency as combining capacity. That NDA item groups elements by equal valency; this one uses the valencies of two ions to fix the molecular formula.
Practice
- practice — not a real PYQ
The correct formula of aluminium sulphate (Al³⁺, SO₄²⁻) is
- (a)AlSO₄
- (b)Al₂(SO₄)₃
- (c)Al₃(SO₄)₂
- (d)Al(SO₄)₃
Answer(b) Al₂(SO₄)₃ — cross the valencies 3 and 2 to balance the charges. - practice — not a real PYQ
The valency of the ammonium ion (NH₄⁺) is
- (a)+1
- (b)+2
- (c)−1
- (d)−2
Answer(a) +1.