Paraphrased: When determining the identity of an unknown carboxylic acid, which is most reliable -- molecular mass, pKa, or melting point? (All three were experimentally calculated).

The molecular mass was determined through a potentiometric titration; the pKa was determined by analyzing the resulting titration curve (pH = pKa at the half-equivalence point, etc.)

I want to say it's definitely not melting point, as impurities in the acid could easily alter that; however, it seems that if something causes the molecular mass to be calculated incorrectly (incorrect molarity of titrated base, incorrect mass of acid, etc.), it would also cause the pKa to be incorrect, so none of them would be more reliable than the other.

I would say the molar mass.

pH isn't that variable (I'm only in Chem 1 though....), and you already said why not the melting point.
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Molar mass
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Molar mass is the least accurate as it doesnt really give you the true identity and is only good when compared to a melting point range. (A molar mass of say 98 can be sulphuric acid or any number of hydrocarbons. Since you dont know its identity, you cant say for sure).

The pKa of a substances is typical to that substance and hence is the most reliable method (if done properly: ie, it matches data tables for a reaction done exactly the same way).
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Last edited by Guitardude19 at Apr 9, 2008,