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NCERT Chemistry Part 1 states that:

"The osmotic pressure method has the advantage over other methods as pressure measurement is around the room temperature and the molarity of the solution is used over molality."

I do not understand how the second statement is really an advantage. Infact, if I had to guess, I would choose molality to be the easier of the two to measure as molarity might involve change in volume upon mixing.

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    $\begingroup$ "My textbook" says...., always cite the name of textbook. Otherwise it's considered as a case of plagiarism. $\endgroup$ – Zenix Mar 15 at 12:36
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    $\begingroup$ @Zenix I am pretty sure the authors have better jobs than to go around suing 17 year olds for posting a question online which quotes their book, but okay $\endgroup$ – ibuprofen Mar 15 at 12:55
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    $\begingroup$ I agree with you, but mentioning the book is $useful$ for other readers. $\endgroup$ – M. Farooq Mar 15 at 14:09
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Molality is used for those people trying to understand how the melting point or the boiling point of water or any other substance, changes when they add a given amount of salt or any other impurity to one kilogram of this pure substance. They don't care about the volume of the obtained solution. They will not keep the solution once obtained. As an example, they just want to check if it is correct to say that adding $1$ mole of a solute to $1$ kg water lowers the freezing point by $1.86°$C, whatever the nature of the solute.

Molarity is used for another group of people. They don't mind about the process of dissolution of a substance in a solvant. They don't care about the volume of water to be used to make $1$ liter of the solution. They just want to know the composition of the solution they have to use, i.e. how many moles (or grams) of a solute are present in a given volume of this solution. If for a given use, they must have a solution containing exactly $9$ g NaCl per liter, they don't care about the way of dissolving the salt. All they need to know is whether this solution can be used for making an injection, or any other use.

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  • $\begingroup$ when measuring concentration in molarity, do we only take the volume of the solvent before adding the solute? do we not care about any volume change which might occur upon adding the solute? $\endgroup$ – ibuprofen Mar 15 at 15:45
  • $\begingroup$ Also molarity changes with temperature, while molality don't. Text cited by OP seems to be ambiguous. $\endgroup$ – Zenix Mar 15 at 15:56
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    $\begingroup$ When preparing a solution with a molarity precisely known, the solid stuff is introduced in a volumetric flask of say 100 mL or 250 mL. Some water is then introduced to about half fill the flask. The flask is stirred until the powder is dissolved. Then water is gradually added up to the graduation limit, so that the total volume of the solution is exactly known. Nobody cares about the exact volume of water to be added. The only important thing is the final volume, which must be known with precision.. $\endgroup$ – Maurice Mar 15 at 17:34

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