Titration.

[cindyy]

Standard Sodium hydroxide solutions cannot be prepared directly from the solids. Explain why.

thankkkkk youuu

[methodsman]

I could be wrong, you may want to double check this but:
- Na (s) is highly reactive with H20 - moisture in the air could react with it

[cindyy]

i thought NaOH was very reactive, but in all states .... hmm

[superflya]

ur right methodsman. and as it absorbs u wont be able to accurately find its mass.

[monokekie]

i thought NaOH was very reactive, but in all states .... hmm

Methodsman is correct

NaOH cannot be used as a primary standard to form Standard Solutions, because primary standards also have to have high molar masses and be unreactive to moisture/other substances present in the air. The solid NaOH may be impure due to the presence of water, so the concentration of NaOH in the solution may be inaccurate and cannot be considered as a standard solution.

Therefore even though NaOH is soluble, it cannot be used as a primary standard.

[methodsman]

Also, adding with caution: - Na is difficult to obtain in pure form (standards should be readily available and cheap).

[cindyy]

(Y) Thanks guys. I hate the theory parts to chem, my brain chooses to not process it!

[cindyy]

wait, the questions says standard solution, so if you have a known concentration, shouldnt that be alright? The high molar mass + availabilty in pure form wouldnt really matter right?

or not ... haha

[naved_s9994]

Not because, as stated its absorbing moisture from atmosphere.

[monokekie]

wait, the questions says standard solution, so if you have a known concentration, shouldnt that be alright? The high molar mass + availabilty in pure form wouldnt really matter right?

A standard solution is a solution that has a an accurately known concentration. But when you dissolve the NaOH in the solution, the concentration cannot be known accurately as the NaOH solid is impure(there's water in it since NaOH is deliquescent) - by absorbing water it is not a primary standard. Hence it is not yet a standard solution, you will need to standardise this NaOH solution possibly through titration.

[cindyy]

alrightio, understood!
thanks

[stonecold]

(Y) Thanks guys. I hate the theory parts to chem, my brain chooses to not process it!

yepp, so true.  curse the questions where you have to write an experimental process!

[Mao]

umm.. I haven't read everything thoroughly, but I think most of the things that have been said are irrelevant. Especially for reactivity: I've touched and handled solid NaOH before (in fact, a whole bucket of NaOH pellets), it's not that bad, it was just a bitch to wash off. Moreso, NaOH has got almost nothing to do with Na(s).

Usually, only primary standards are prepared from using an accurate mass. NaOH is not a primary standard because solid NaOH absorbs water (as moisture in air), it also absorbs CO2 in the atmosphere to form hydrogen carbonate.

Hence, standard (read: accurate concentration) NaOH cannot be prepared directly from solid, a primary standard must be prepared first and titrated against the NaOH prepared from solid to work out the accurate concentration. I.e. you must standardize the NaOH solution before it can be considered a standard solution. Compare this to primary standards, which do not require the standardization step since their accurate concentration can be calculated directly from the mass of the solids dissolved. The concentration of NaOH can sometimes be 10% lower than expected value (i.e. you expect a 0.1M solution and it turns out to be 0.09M)

[cindyy]

Yeah, i thought it was a little irrelevant as well so i only put the fact that NaOH absorbs water + CO2 in the air in my answer! (Y)

[monokekie]

standardise or standardize? :p