molecular weight
Noun 1. (Chemistry) A number equal to the sum of the atomic weights of all the atoms in a molecule. It is a dimensionless quantity representing the average mass of a molecule of that substance relative to one-twelfth the mass of an atom of carbon-12. It is often expressed in atomic mass units (amu) or Daltons (Da).
The term "molecular weight" is used to specify the mass of a single molecule of a compound. It is a fundamental property in chemical calculations. * The molecular weight of water (H₂O) is approximately 18.015 amu. * Knowing the molecular weight is essential for preparing solutions with a specific molar concentration. * The compound's high molecular weight contributes to its low volatility.
- Average Molecular Weight: For polymers or mixtures of isotopes, the term often refers to the average molecular weight, which is a weighted mean based on the distribution of different molecular masses in the sample.
- The average molecular weight of the polymer sample was determined by gel permeation chromatography.
- Formula Weight: For ionic compounds (like NaCl) which do not exist as discrete molecules, the analogous term "formula weight" is often used, though "molecular weight" may be informally applied.
- Molecular Mass: A term often used synonymously with molecular weight. Strictly, molecular mass is the mass of a single molecule measured in unified atomic mass units (u), while molecular weight is a dimensionless ratio.
- Molar Mass: A closely related but distinct concept. Molar mass is the mass of one mole of a substance and is expressed in grams per mole (g/mol). Numerically, it is equal to the molecular weight (in amu) but has different units.
- The molar mass of water is 18.015 g/mol.
- Relative molecular mass (Mᵣ)
- Molecular mass (in common usage)
While "molecular weight" is a standard term, in modern scientific practice, the terms "relative molecular mass" (Mᵣ) and "molar mass" are often preferred for greater precision. "Molecular weight" remains in widespread use, particularly in educational and applied contexts.
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(chemistry) the sum of the relative atomic masses of the constituent atoms of a molecule
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