[xstrae]

I've been reading a bit on normality. I came across that the number of replacable atoms in Phosphoric (H3PO4) is 3 and in phosphoric acid (H3PO3). Came someone explain why? Thanks!

[Dan]

I google imaged h3po4, this was the second result

http://www.800mainstreet.com/acid_base/h3po4-labelled.gif

[Donaldson Tan]

It's a tribasic acid. It contains 3 -OH functional groups which is capable of releasing protons.

[xstrae]

Thanks.. one more doubt though.
My teacher says the number of replaceable hydrogen atoms in H3PO3 is only 2. I google imaged it :

http://www.wuzhouchem.com/cataloged/inor/phosphorous_acid.gif
 
Why is it only 2? Is it because the double bond with phosphorus makes the 3rd Hydrogen atom harder to remove?

[swati]

Thanks.. one more doubt though.
My teacher says the number of replaceable hydrogen atoms in H3PO3 is only 2. I google imaged it :

http://www.wuzhouchem.com/cataloged/inor/phosphorous_acid.gif
 
Why is it only 2? Is it because the double bond with phosphorus makes the 3rd Hydrogen atom harder to remove?

As you can view in that picture that there are just 2 -OH bonds . And O being highly electronegative will facilitate the release of H⁺ whereas since there is not much electronegativity difference between P and H , so it is difficult to break bond   P -- H  and thus release of 3rd H⁺ is not easy

[xstrae]

so the double bond is of no significance here? anyway thanks swati

[swati]

so the double bond is of no significance here?

The double bonded O does not have any significance for counting the number of replacable hydrogen atoms .

anyway thanks swati

welcome 

[sdekivit]

the Ka of HPO4(2-) is 4,8 x 10^-13, while Kb of HPO4(2-) = 1,6 x 10^-7

--> HPO4(2-) is a stronger base then an acid, so HPO4(2-) will hardly lose its third proton.

[Borek]

the Ka of HPO4(2-) is 4,8 x 10^-13, while Kb of HPO4(2-) = 1,6 x 10^-7

--> HPO4(2-) is a stronger base then an acid, so HPO4(2-) will hardly lose its third proton.

OP asked about H₃PO₃, not H₃PO₄.

But even if you refer to H₃PO₄ you are wrong. What will be ratio of PO₄^3-/HPO₄^2- concentrations at pH=12.4 (hint: HH equation)? You still think it is "hardly lost" proton?

[sdekivit]

yes if you make the pH to 12,4 you're right, but if you drop H3PO4 or H3PO3 in water you hardly find any PO4(3-) or PO3(3-) in your solution, because the third dissociation won't take place !

(i don't have time to prove that right now)

by the way: i have abolished HH-equation in my life --> i prefer the normal equilibrium expression.

While the molecule itself has 3 dissociable protons, it rarely occurs that the third one is dissociated. (even the dissociation of the second will be a problem when only putting H3PO4 in water)

[Borek]

yes if you make the pH to 12,4 you're right, but if you drop H3PO4 or H3PO3 in water you hardly find any PO4(3-) or PO3(3-) in your solution, because the third dissociation won't take place !

Please read thread subject - it is not about concentrations, but about number of protons that CAN dissociate. Besides, stating that 3^rd dissociation step won't take place is simply erroneous - it will take place and it will be described by the dissociation constant you have already posted.

by the way: i have abolished HH-equation in my life --> i prefer the normal equilibrium expression.

And? I am using whichever makes expanation or solving of the question easiest. Sometimes it is overal dissociation equation, sometimes stepwise equation, sometimes protonation equation - and sometimes HH. Does it change the result?

OP question is ambiguous - there are two isomers of H₃PO₃ - one has two protons that can dissociate, one has three.

[sdekivit]

Does it change the result?

i never said or will ever say it would change the result  i only said i never use HH 

[Will]

OP question is ambiguous - there are two isomers of H₃PO₃ - one has two protons that can dissociate, one has three.

Are you saying the P(III) version of H₃PO₃ actually exists? I'm confusedl; I thought only the diprotic one existed.

[Borek]

I did some digging and it seems it depends on the source. Some sources give only diprotic form (my inorganic chemistry book, in Polish), some suggest triprotic form exists (like emolecules, formerly chmoogle, http://www.emolecules.com/cgi-bin/search?t=ex&q=OP%28O%29O), wikipedia states these are tautomeric forms (with diprotic form dominating).

[AWK]

Concerning structure of H3PO3:
In crystalline state it show structure H2PHO3 (neutron difraction for D2PDO3).
During reaction of PCl3 with RONa you can obtain P(OR)3 which after hydrolysis forms H2PHO3.
H2PHO3 react with only 2 molecules of NaOH