[priyanka]

Hi,
  could you please help me find an answer to these question?
  Why cant std. solution of Na OH be prepared by direct weighing?
  Why does Na OH exposed to air become a liquid and then into white powder?

[Borek]

Think what substances are present in the air (apart from the three most abundant gases).

[priyanka]

 Sorry sir but i really couldnt think of an answer.

[Borek]

OK, LIST most abundant substances present in the air. That doesn't require thinking, just check it somewhere.

[priyanka]

 oxygen, nitrogen, water vapour, carbon di oxide and argon.
 Well, i do know that Na OH is hygroscopic and hence it turns into liquid when exposed to air but why does it turn into a white powder when left for a long time?

[Borek]

Is there any gas - between those listed - that can react with a strong base?

[priyanka]

 Yes, i think i know the answer. Carbon dioxide does react with Na OH to form sodium carbonate. Then, this reacts with water and carbondioxide to form sodium bicarbonate. am i right?
 Is the answer to my first question similar to this?
           

[Borek]

You see - it wasn't that hard.

Now, when it comes to the first question - if you take 1g of NaOH (note: no space in the formula) that was exposed to air, can you tell how much NaOH it really contains?

[skyjumper]

I weigh out NaOH stock solutions.... Im a bad person

[Borek]

I weigh out NaOH stock solutions.... Im a bad person

Actually weighting stock solution is OK. But I agree you are a bad person, you are interfering with the help 

[priyanka]

Well, i guess it becomes difficult to do. Thanks a ton for the help.