[becchino]

Hello friends..

WhyPharmacopoeia asserts that to prepare a solution of NaOH 1m...

1 M Sodium hydroxide. 3006300.
Dissolve 42 g ofsodium hydroxide R in carbon dioxide-free
water R and dilute to 1000.0 ml with the same solvent.
 
42? Not 40?

[Borek]

40 IMHO.

Perhaps they try to take into account fact NaOH is never exactly dry. But if you need a 1 M solution, using 40 g should be accurate enough. If you need 1.00 M solution, you should prepare it in a different way, checking concentration produced and then adjusting it.

[becchino]

2 g of Water?!?? 

[Arkcon]

2 g of Water?!?? 

That does sound silly, you're right.  It seems almost certain to be a typo.  The NaOH pellets do get wet quickly, so I always work as quickly as I can.  Someday, I just might weigh out 40.0 g and see how quickly to absorbs 2 g worth of moisture.

[Enthalpy]

Do the mol/L refer to the volume of the initial pure water, or to the volume of the solution? Wiki claims "volume of the mixture"
http://en.wikipedia.org/wiki/Molarity

[becchino]

Do the mol/L refer to the volume of the initial pure water, or to the volume of the solution? Wiki claims "volume of the mixture"
http://en.wikipedia.org/wiki/Molarity

Sorry I do not understand your question

[Corribus]

He's asking about whether the unit "mole per liter" refers to "mole per liter of solvent" or "mole per liter of solution". For sufficiently dilute solutions, these two possibilities are functionally equivalent. But to be rigorous, the solute takes up volume as well, and as the relative concentration increases, the volume of solute becomes more important. Molarity usually defines as "mole of solute per liter of solution".

[Arkcon]

Of course, there is another unit -- molality, abbreviated as m, which is moles per kg of solution.  So that could be what this formula is trying to make.

[Borek]

5% water in pellets doesn't sound ridiculous to me.

Not that I have any reliable information.

[DrCMS]

Aldrich sell NaOH at >97% (but KOH is only >85%) so a sample with 5% water is plausible. 

However the method should give a reason why they use the extra.