Molecular mass — correct modern definition: “The molecular mass of a substance is the number of times a molecule is heavier than the hydrogen atom.” Evaluate this statement for correctness under the modern standard.

Introduction / Context:
Precise definitions matter in chemistry and thermodynamics. The historical hydrogen-based scale has been superseded; today, molecular (or relative molecular) mass is referenced to the carbon-12 isotope standard. This distinction affects how we define atomic and molecular masses in SI.

Given Data / Assumptions:

• Modern IUPAC definition using the carbon-12 scale.
• Relative molecular mass is dimensionless; molar mass uses g/mol or kg/mol.
• Hydrogen exists as isotopes (protium, deuterium, tritium), complicating a hydrogen-based reference.

Concept / Approach:
The modern standard defines the atomic mass unit (u) so that the mass of a carbon-12 atom is exactly 12 u. Relative molecular mass M_r of a molecule is the ratio of its mass to 1/12 of the mass of a carbon-12 atom, not to the mass of a hydrogen atom. Therefore, the statement comparing to hydrogen is outdated and incorrect today.

Step-by-Step Solution:
Identify the claim: “heavier than the hydrogen atom.”Recall standard: 1 u corresponds to 1/12 of carbon-12 mass; molecular masses are sums of atomic relative masses on this scale.Conclude that the hydrogen comparison does not define molecular mass under modern conventions.

Verification / Alternative check:
Periodic tables list relative atomic masses on the carbon-12 scale; calculated molecular masses (e.g., H2O ≈ 18.015) are consistent with this standard, not with hydrogen-based scaling.

Why Other Options Are Wrong:
Marking “True” would accept an obsolete definition and conflict with SI/IUPAC standards.

Common Pitfalls:
Confusing molar mass (kg/mol) with relative molecular mass (dimensionless); citing historical hydrogen scale without noting its replacement.

Final Answer:
False