Posted by Aman on .
The normality of 0.3 M phosphorus acid (H3PO3) is.......
Normality is Molar concentration divided by an equivalence factor. (n = c/f)
The hydrogen equivalence factor of an acid is the number of moles of acid that contains 1 mole of acidic hydrogen atoms.
The hydrogen equivalence factor of a triprotic acid is 1/3, so the normality of a triprotic acid is 3 times its molar concentration.
So for phosphorus acid: 0.3M = 0.9N
Any confusion felt during the reading of this explanation is one of the reasons the use of normality is depreciated in chemistry.
(Unfortunately it is still sometimes used in medicine.)
H3PO3 is diprotic, not triprotic like H3PO4.
I most certainly disagree with any statement that normality is not needed anymore although I agree it is confusing (sometimes) and its use has been discouraged since the IUPAC and a lot of smarter people than I am changed all the rules. (Yes, you hit a sore spot. I wish we still had normality.)
Second, I would like to point out to Aman that technically the question posed can't be answered without knowing the equation. Note that
H3PO3 + NaOH ==> NaH2PO3 + H2O and the factor is molar mass H3PO3/1 but for
H3PO3 + 2NaOH ==> Na2HPO3 + 2H2O the factor is molar mass H3PO3/2. That is the equivalent weight of a substance is the molar mass divided by the number of REPLACED H ions. Therefore, an acid like H3PO4 may have three different equivalent weights depending upon how many H ions are actually displaced in a reaction.