John Burgess wrote:
>  anhydrous OA has a molecular weight of 45 and the dihydrate 63. 

My handbook of Chemistry and Physics list OA's molecular weight as 
90.04. It is ethanedoic acid, HO2CCO2H or 2 carbon (24), 2 Hydrogen (2) 
and 4 Oxygen (64) which gives you 90.

Formic acid is about 45, so maybe you are thinking about it instead? 
Plus, if you add 2 water molecules you would get a MW of 45+36=71 for OA 
dihydrate (but the dihydrate is listed as 126, which makes sense  90+36).

Bill Truesdell
Bath, Maine

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