An element Z has a combining power of 3. Which of the following formulae is correct of its compound?

Analyze the question: Element Z has a combining power (valency) of 3. We need the formula that correctly reflects Z with valency 3.

Concept: Combining power (valency) tells us how many atoms of another element one atom combines with. For the compound to be correct, the total positive charge must equal the total negative charge (criss-cross method). Z has a valency of 3, so it behaves as Z³⁺.

Check each option:

• A. ZS3: Sulphur (S²⁻) has valency 2. Z³⁺ with S²⁻ should give Z₂S₃. ZS3 would need Z to have a valency of 6, so it is wrong.
• B. ZSO3: SO₃²⁻ (sulphite) has a charge of 2. Z³⁺ with SO₃²⁻ gives Z₂(SO₃)₃. ZSO₃ would need Z²⁺, so it is wrong.
• C. ZSO4: SO₄²⁻ has a charge of 2. This would need Z²⁺, so it is wrong. Correct is Z₂(SO₄)₃.
• D. Z2O3: Z³⁺ and O²⁻ criss-cross to give Z₂O₃. Charge check: 2(+3) = +6 and 3(−2) = −6. Balanced, so this is correct. (The option is written as "Z203", which is Z₂O₃.)
• E. Z3Cl2: Cl⁻ has valency 1. Z₃Cl₂ would need Z to have a valency of 2/3, so it is wrong. Correct is ZCl₃.

Answer: D. Z₂O₃.

Tip: Use the criss-cross method. Write the valencies, swap them, and use them as subscripts. For Z (3) and O (2): Z₂O₃. Then always check that total positive charge equals total negative charge.