Which of the following pairs of compounds contain the same total number of atoms per molecule?

Question

Examoogle · Accepted Answer

The correct answer is Option B: H ₂ SO ₄ and N ₂ O ₅. To determine the number of atoms per molecule, we sum the subscripts of the elements present in the chemical formula. Option B is correct because both $H_{2}SO_{4}$ and $N_{2}O_{5}$ consist of exactly 7 atoms per molecule.

Step-by-Step Solution & Concepts:
To determine the number of atoms per molecule, we sum the subscripts of the elements present in the chemical formula. Option B is correct because both $H_{2}SO_{4}$ and $N_{2}O_{5}$ consist of exactly 7 atoms per molecule.

Working Principle: The atomicity of a molecule is defined by the total count of constituent atoms represented by the subscripts in its chemical formula. In $H_{2}SO_{4}$, we count 2 atoms of H, 1 atom of S, and 4 atoms of O (2 + 1 + 4 = 7). In $N_{2}O_{5}$, we count 2 atoms of N and 5 atoms of O (2 + 5 = 7).

Key Formulas: $Total Atoms = \sum Subscripts$

Key Points: Atomicity represents the total number of atoms present in one molecule of an element or compound.; The subscript after an element symbol denotes the number of atoms of that specific element in the chemical unit.; When no subscript is written, the value is implicitly 1.

Advantages: Simple method to identify molecular composition; Essential for balancing chemical equations

Disadvantages: Does not account for molecular structure or arrangement; Only applicable to covalent/molecular substances

Applications: Stoichiometry calculations; Determination of molar mass

Additional Info: Option A: $NH_{3}$ (1+3=4) and $CH_{4}$ (1+4=5).. Option C: $CO_{2}$ (1+2=3) and $H_{2}O_{2}$ (2+2=4).. Option D: $C_{2}H_{6}O$ (2+6+1=9) and $PCl_{3}$ (1+3=4).

Answer: B is correct — Both $H_{2}SO_{4}$ and $N_{2}O_{5}$ contain 7 atoms per molecule.

Exam Tip: Always verify if a subscript is missing (e.g., in $CO_{2}$, C is implied $C_{1}$); missing a subscript is a common trap in competitive exams.

Examoogle · Answer

Option A: NH ₃ and CH ₄

Examoogle · Answer

Option C: CO ₂ and H ₂ O ₂

Examoogle · Answer

Option D: C ₂ H ₆ O and PCl ₃

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