How is it flipped in 2nd step ?

The number of moles is inversely proportional to the molar mass.

@KJW What is that have to do with ?

I meant Normal = Calculated and Observed=Abnormal (has the definition below it says)

5 hours ago, HbWhi5F said:

@KJW What is that have to do with ?

You asked a question and I answered it.

5 hours ago, HbWhi5F said:

I meant Normal = Calculated and Observed=Abnormal (has the definition below it says)

Yes, I read what was written. My answer stands.

@KJW I don't get the context of "The number of moles is inversely proportional to the molar mass." in van't hoff's factor.

The definition/formula in book seems self-contradictory.

9 hours ago, HbWhi5F said:

The definition/formula in book seems self-contradictory.

There is nothing wrong with the excerpt from the book.

9 hours ago, HbWhi5F said:

@KJW I don't get the context of "The number of moles is inversely proportional to the molar mass." in van't hoff's factor.

Do you know what "inversely proportional" means?

Perhaps the following will help:

[math]i = \dfrac{\text{Calculated molar mass}}{\text{Observed molar mass}}[/math]

[math]i = \dfrac{\text{Observed number of moles}}{\text{Calculated number of moles}}[/math]

[math]i = \dfrac{\text{Observed colligative property}}{\text{Calculated colligative property}}[/math]

[If the above LaTeX doesn't render, please refresh browser.]

Edited February 10Feb 10 by KJW

19 hours ago, HbWhi5F said:

@KJW I don't get the context of "The number of moles is inversely proportional to the molar mass." in van't hoff's factor.

The definition/formula in book seems self-contradictory.

I think it may be expressing this in terms of molar mass that is causing you the confusion. Think first of all in terms of the numbers of molecules, i.e. the number of moles of substance. Then i makes obvious sense as the ratio of the "effective" number of moles of particles (i.e. how the substance actually behaves in practice) to the "theoretical" number of moles of the substance (i.e. as it appears on paper from the chemical formula).

I must say I think the idea of "abnormal molar mass" is a fairly unhelpful way of thinking about what that is going on. It could almost be designed to get you in a muddle. Nevertheless, see if the following explanation helps. For example if you consider a solution of a partially dissociated substance, its effective molarity would be less than the theoretical value. But the number of moles of a substance = mass of substance present/molar mass. So then an "effective" molarity < "theoretical" molarity could be interpreted to mean the "effective" molar mass > "theoretical" molar mass.

This I think is what @KJW is getting at.

Personally, I struggle to think of any situation in which the concept of an "abnormal molar mass" is at all useful. (But if other readers can think of a use for it, I'll be interested to learn). I much prefer to leave the molar mass as standard, and think of effective concentrations differing from that predicted by the (standard) molar mass.

Edited February 10Feb 10 by exchemist

Start with the molar mass.

This is determined by the stoichiometry of chemical reactions. That is it depends completely on chemical composition.

A different value for the molar mass may be found when another (usually physical not chemical) method is used to calculate or measure it.

This different value is called the abnormal mass.

The difference arises because some but not all ( eg colligative properties,) physical effects are due to the number of molecules, not their composition.

Changes in composition may arise due to low abnormal molar weight (due to association) or high abnormal mass (due to dissociation).

This is a clear explanation with good examples.

https://www.chemistrystudent.com/ncert-class-12/1-solutions/abnormal-molar-masses.html

Edited February 10Feb 10 by studiot

3 hours ago, studiot said:

Start with the molar mass.

This is determined by the stoichiometry of chemical reactions. That is it depends completely on chemical composition.

A different value for the molar mass may be found when another (usually physical not chemical) method is used to calculate or measure it.

This different value is called the abnormal mass.

The difference arises because some but not all ( eg colligative properties,) physical effects are due to the number of molecules, not their composition.

Changes in composition may arise due to low abnormal molar weight (due to association) or high abnormal mass (due to dissociation).

This is a clear explanation with good examples.

https://www.chemistrystudent.com/ncert-class-12/1-solutions/abnormal-molar-masses.html

Interestingly, all the references to this "abnormal molar mass", including the one you cite, seem to be from Indian source material. I don't recall it from my own undergraduate studies (in the UK), there is no reference to it in my old Moore Physical Chemistry, nor it seems in the Chemistry Libretexts, or in Wikipedia or the other sources I habitually consult when revising chemistry.

Edited February 10Feb 10 by exchemist

1 hour ago, exchemist said:

Interestingly, all the references to this "abnormal molar mass", including the one you cite, seem to be from Indian source material. I don't recall it from my own undergraduate studies (in the UK), there is no reference to it in my old Moore Physical Chemistry, nor it seems in the Chemistry Libretexts, or in Wikipedia or the other sources I habitually consult when revising chemistry.

I had not come across "abnormal molar mass" in my studies, either. However, the excerpt from the book in the OP made it clear what "abnormal molar mass" meant, and this was the basis of my reply.

10 minutes ago, KJW said:

I had not come across "abnormal molar mass" in my studies, either. However, the excerpt from the book in the OP made it clear what "abnormal molar mass" meant, and this was the basis of my reply.

Oh yes I don’t mean to suggest it’s wrong or anything like that. Just a bit weird - and in my opinion of doubtful utility to someone trying to learn the principles of chemistry at school level.