molecular weight: meaning, definition, pronunciation and examples
TechnicalFormal, scientific
Quick answer
What does “molecular weight” mean?
The mass of a single molecule of a substance, expressed in atomic mass units (amu) or Daltons (Da). It is calculated by summing the atomic weights of all atoms in the molecule's chemical formula.
Audio
Pronunciation
Definition
Meaning and Definition
The mass of a single molecule of a substance, expressed in atomic mass units (amu) or Daltons (Da). It is calculated by summing the atomic weights of all atoms in the molecule's chemical formula.
1. In biochemistry and polymer science, often used interchangeably with 'molecular mass' or 'molar mass' (though molar mass refers to mass per mole). 2. A key parameter for characterizing polymers, proteins, and other large molecules, influencing properties like diffusion and viscosity.
Dialectal Variation
British vs American Usage
Differences
No significant lexical or definitional difference. British texts may slightly prefer 'relative molecular mass' for precision, but 'molecular weight' is dominant in both.
Connotations
Neutral scientific term in both.
Frequency
Equally frequent in academic/technical contexts in both varieties.
Grammar
How to Use “molecular weight” in a Sentence
The molecular weight of [NOUN PHRASE] is [NUMBER].[NOUN PHRASE] has a molecular weight of [NUMBER].to calculate/determine the molecular weight of [NOUN PHRASE].Vocabulary
Collocations
Examples
Examples of “molecular weight” in a Sentence
adjective
British English
- The high-molecular-weight fraction was isolated.
- They used a low-molecular-weight marker.
American English
- The high molecular weight polymer was synthesized.
- A low-molecular-weight standard was run.
Usage
Meaning in Context
Business
Rare; might appear in R&D reports, pharmaceutical patents, or material specifications.
Academic
Core term in chemistry, biochemistry, pharmacology, and materials science papers and textbooks.
Everyday
Virtually never used.
Technical
Fundamental term in laboratory protocols, analytical data sheets, and quality control documents.
Vocabulary
Synonyms of “molecular weight”
Strong
Neutral
Weak
Vocabulary
Antonyms of “molecular weight”
Watch out
Common Mistakes When Using “molecular weight”
- Using 'molecular weight' when referring to the mass of a mole of substance (that is molar mass, unit g/mol).
- Adding incorrect units; it is a relative, dimensionless quantity, though Daltons are commonly used.
- Confusing it with 'atomic weight'.
FAQ
Frequently Asked Questions
No, but they are related. Molecular weight (or relative molecular mass) is dimensionless. Molar mass has units of mass per mole (e.g., g/mol) and is numerically equal to the molecular weight when expressed in Daltons.
By summing the standard atomic weights (from the periodic table) of all the atoms in a molecule's chemical formula. For H₂O: (2 x 1.008) + 15.999 = 18.015 Da.
Polymers are chains of varying lengths. The average molecular weight (like number-average or weight-average) determines key material properties such as strength, melting point, and viscosity.
Strictly, ionic compounds like NaCl do not exist as discrete molecules in a crystal, so the term 'formula weight' (sum of atomic weights in the empirical formula) is preferred, though 'molecular weight' is often used loosely.
The mass of a single molecule of a substance, expressed in atomic mass units (amu) or Daltons (Da). It is calculated by summing the atomic weights of all atoms in the molecule's chemical formula.
Molecular weight is usually formal, scientific in register.
Molecular weight: in British English it is pronounced /məˈlek.jə.lə weɪt/, and in American English it is pronounced /məˈlek.jə.lɚ weɪt/. Tap the audio buttons above to hear it.
Learning
Memory Aids
Mnemonic
Think of a molecule on a tiny scale. Its 'molecular weight' is like the sum of the 'weights' (atomic masses) of all the individual atom 'friends' holding hands inside it.
Conceptual Metaphor
WEIGHT AS A SUM (The molecule is a collection, and its weight is the total of its parts).
Practice
Quiz
What is the most precise synonym for 'molecular weight' in a formal chemical context?