Why is h3po3 diprotic and not triprotic

The first dissociation constant is necessarily greater than the second i. For example, sulfuric acid H 2 SO 4 can donate two protons in solution:. This first dissociation step of sulfuric acid will occur completely, which is why sulfuric acid is considered a strong acid; the second dissociation step is only weakly dissociating, however. Take, for example the three dissociation steps of the common triprotic acid phosphoric acid:.

For example, a generic diprotic acid will generate three species in solution: H 2 A, HA — , and A 2- , and the fractional concentration of HA — , which is given by:. The following formula shows how to find this fractional concentration of HA — , in which pH and the acid dissociation constants for each dissociation step are known:.

Fractional ion calculations for polyprotic acids : The above complex equations can determine the fractional concentration of various ions from polyprotic acids. Solve equilibrium problems using the appropriate approximations for weak and strong polyprotic acids. Polyprotic acids can lose more than one proton. When determining equilibrium concentrations for different ions produced by polyprotic acids, equations can become complex to account for the various components.

For a diprotic acid for instance, we can calculate the fractional dissociation alpha of the species HA — using the following complex equation:.

Equation for finding the fractional dissociation of HA- : The above concentration can be used if pH is known, as well as the two acid dissociation constants for each dissociation step; oftentimes, calculations can be simplified for polyprotic acids, however.

We can simplify the problem, depending on the polyprotic acid. The following examples indicate the mathematics and simplifications for a few polyprotic acids under specific conditions. Because the first dissociation is so strong, we can assume that there is no measurable H 2 SO 4 in the solution, and the only equilibrium calculations that need be performed deal with the second dissociation step only.

At a pH equal to the pK a for a particular dissociation, the two forms of the dissociating species are present in equal concentrations, due to the following mathematical observation. Take for instance the second dissociation step of phosphoric acid, which has a pK a2 of 7. As long as the pK a values of successive dissociations are separated by three or four units as they almost always are , matters are simplified. Phosphoric acid : The chemical structure of phosphoric acid indicates it has three acidic protons.

When a weak diprotic acid such as carbonic acid, H 2 CO 3 , dissociates, most of the protons present come from the first dissociation step:. Privacy Policy. Skip to main content. Acids and Bases. The trianion shown below doesn't have the benefit of resonance stabilization. This makes removing the third proton much harder.

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