[tonydoss76]

Hi,

I need to make a stock solution of 0.4M oxalic acid from oxalic acid dihydrate salts. To calculate the mass of salts I need, do I use the molecular weight for oxalic acid or oxalic acid dihydrate? I would say the latter but some of my colleagues (not chemists, me neither) use the latter!?
thanks
Tony

[Arkcon]

Your instincts are probably correct, but there are caveats.  First of all, do you have the dihydrate?  What does the bottle say?  Is oxalic acid dihydrate effervescent -- that is, does it lose its own water over time?  What does the method say?  If its been done wrong for a long time, and gives the answer you want, you can't just change to make it correct, unfortunately.  You will have to verify that the method works the same. 

What is, in fact, the difference in mass between solutions made with both molecular weights -- maybe the difference is negligible, or beyond your ability to measure?  You can compute it both ways, on your own, as a check.

[tonydoss76]

thanks. I wouldn't trust what others are doing, because they're not chemists either so it's likely they got it wrong. What I'd like to know is what is chemically accurate. I've computer the difference and it is significant. Does anyone have an idea what is chemically correct? Use the molecular weight of the oxalic acid or the oxalic acid dihydrate?

[Borek]

As Arkcon already wrote - that's not something that can be easily answered yes or no.

In general, especially when aiming at solution of known molar concentration, you should use the molecular mass of the dihydrate, as this way you will use correct number of moles of the acid (that's what counts). But it still doesn't guarantee the concentration will be the one you need.