Q.38 Which of the following statements is true?
1. Molecular weight is expressed in Daltons.
2. Molecular mass is expressed in Daltons.
3. Relative molecular weight is expressed in Daltons.
4. Relative molecular mass is expressed in Daltons.
Molecular weight and related terms describe the mass of chemical entities, with “Daltons” (Da) as the standard unit for atomic and molecular masses on the atomic scale, equivalent to 1/12 the mass of carbon-12.
Correct Answer
1. Molecular weight is expressed in Daltons.
Molecular weight (MW) is the sum of atomic masses of a molecule’s atoms, numerically expressed in daltons (Da or u)—a dimensionless unit scaled to the atomic mass constant.
Option Analysis
Molecular Weight in Daltons (Option 1)
Correct: MW is conventionally given in Da (e.g., H2O MW = 18.015 Da), distinguishing it from molar mass in g/mol.
Molecular Mass in Daltons (Option 2)
Molecular mass refers to a single molecule’s mass (in Da), but “molecular weight” is the standard term used in this context; subtle distinction, but option 1 prevails.
Relative Molecular Weight in Daltons (Option 3)
Incorrect: Relative molecular weight/mass (Mr) is a pure ratio (dimensionless number, e.g., 18 for water), not assigned Da units.
Relative Molecular Mass in Daltons (Option 4)
Incorrect: Relative molecular mass (Mr) is also dimensionless; Da applies to absolute masses, not relatives.
| Option | Expressed in Daltons? | Units/Definition |
|---|---|---|
| Molecular weight | Yes | Absolute mass sum (Da) |
| Molecular mass | Sometimes | Single molecule (Da) |
| Relative molecular weight | No | Dimensionless ratio |
| Relative molecular mass | No | Dimensionless ratio |
Clinical Relevance
Precise MW in Da is key for biochemistry (e.g., protein sizing via SDS-PAGE) and exams like CSIR-NET, where g/mol vs. Da confusion is common.
