Difference between molecular mass and molar mass

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Discussion Overview

The discussion revolves around the concepts of molecular mass and molar mass, exploring their definitions, differences, and the terminology used in chemistry. Participants engage in clarifying these terms and their implications in various contexts.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants assert that the molecular mass of water is constant at 18 g for one mole, while molar mass varies with the number of moles considered.
  • There is a contention regarding the terms molecular mass and molecular weight, with some arguing they are synonymous, while others clarify that weight involves gravitational factors.
  • One participant notes that "molar mass" refers specifically to the mass of one mole of a substance, emphasizing the distinction from molecular mass.
  • A participant mentions that "molecular weight" is an outdated term, suggesting that it should be replaced with "relative molecular mass" based on specific mass standards.
  • Another participant challenges the definition of "gram mole," arguing that it is not simply molecular mass times one gram, and explains the relationship between molecular mass in Daltons and molar mass in g/mol.
  • There are multiple references to the historical context of terminology, indicating that some terms have persisted despite evolving definitions in the scientific community.

Areas of Agreement / Disagreement

Participants express differing views on the definitions and relationships between molecular mass, molar mass, and molecular weight. There is no consensus on the terminology, and the discussion remains unresolved regarding the clarity and usage of these terms.

Contextual Notes

Some participants highlight the confusion arising from the interchangeable use of terms like molecular mass and molar mass, as well as the historical context of these terms. The discussion reflects varying interpretations and definitions that may depend on specific contexts or standards.

gracy
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Molecular mass of water is constant i.e mass of one mole of water i.e 18 g
Molar mass is the mass of a substance depending on how many moles of substance is taken into consideration.
Ex.
A mole of H2O: 18 g
Two moles of H2O: 36 g
Three moles of H2O: 54g
and so on.
Right?
And molecular mass and molecular weight are same ,right?
 
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gracy said:
molecular mass and molecular weight are same ,right?
Not really. Mass is mass. Weight is mass times acceleration. When you're in the lab "weighing" materials you are performing that operation in the Earth's gravitational field and the acceleration is 9.8 m/s2 and usually ignored in scale or balance calibrations.
gracy said:
Molar mass is the mass of a substance depending on how many moles of substance is taken into consideration.
No. "Molar mass" is the mass of ONE mole. Chemists usually use the "gram mole" which is molecular mass times one gram.
gracy said:
mass of one mole of water i.e 18 g
Yes.
gracy said:
Molecular mass of water is constant
Yes.
 
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Molecular weight is an antiquated term that, back in the day, was used to describe what we now call molecular mass. (I still call it molecular weight)

Chet
 
Chestermiller said:
what we now call molecular mass
And, to be strictly correct, "relative molecular mass" with a specification of which mass standard 16O, old standard, or 12C, new.
 
And what about difference between molecular mass and molar mass?
 
Bystander said:
Chemists usually use the "gram mole" which is molecular mass times one gram.
 
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Bystander said:
Chemists usually use the "gram mole" which is molecular mass times one gram.
I must be an unusual chemist, then :-)
Whatever a "gram mole" may be, it is certainly not the molecular mass times one gram.
Rather the molecular mass, i.e. the mass of a single molecule, is usually specified in units of "Daltons" where 1 u = 1 g/mol /Na =1.660 538 921(73)×10−27 kg.
This has the effect that the molar mass, i.e. mass of one mole of these molecules has the same numerical value in units of g/mol as has the molecular mass in units of Daltons.
This has the confusing effect for adepts that chemists often don't differentiate between molecular and molar mass.
Also, they often speak of "molecular weight" instead of molecular mass, a relic from the pre-history of science, when people still did not clearly differentiate between mass and weight. E.g. Berzelius had a wooden sign over the door of his lab, saying:
"God has ordered everything according to its number, measure and weight".
But till then, almost 200 years have passed.
 
To Bystander
Why you have blocked me?You are my favorite.You have been very helpful to me.
Regards.
 
gracy said:
Why you have blocked me.
Huh?
 
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To Bystander
Why have you blocked me?You are my favorite.You have been very helpful to me.
Regards.
 
  • #11
gracy said:
Why you have blocked me?
I haven't blocked anyone as far as I know.
 
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Bystander said:
I haven't blocked anyone as far as I know.
Why I am not able to do conversation with you?I have asked this question,and one of members answered that you might have blocked me.
 
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There are limits on number of conversations. Did have a couple old ones that I just cleared out. Try again.
 
  • #14
Bystander said:
Try again.
I am still not able to start conversation with you.
 

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