Is H3PO4 a strong or weak acid

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The evil phosphoric acid

10 ml of 0.1 mol / l phosphoric acid are given.

A laboratory assistant makes the following consideration: Phosphoric acid has the formula H.3PO4, and since it is a tri-protonic acid, the pH value is calculated according to pH = -log 0.3.

Basically for phosphoric acid, as for every acid, the rules for calculating the pH value, which are summarized again below, apply. What complicates matters here (seemingly) is the fact that phosphoric acid is a tri-protonic acid. A tri-protonic acid, however, is nothing else than an acid in which the dissociation of the first stage, apart from the oxonium ion H3O+ Another acid is formed, here the dihydrogen phosphate ion, H2PO4-. This ion can release a proton in water, and the hydrogen phosphate ion (HPO42-). As you can see, this ion also has a proton, so it is also an acid.
It is well known that the release of the positive proton to a base (here water) constitutes the acidic property and will become increasingly difficult in the order of the acids mentioned above. (An uncharged particle like the H3PO4 can give off a positive proton more easily than a particle that is already negatively charged like the H2PO4-). This is also reflected in the acid constants.

In this way, the concentration of the oxonium ions in the aqueous solution of a multi-protonic acid is primarily determined by the protolysis of the first stage. Firstly, the second stage is always a weaker acid than the first and, secondly, the protolysis of the second stage is suppressed by the protons that were formed from the first stage. (Principle of Le Chatelier). This applies in particular to medium-strength and weak acids, to which phosphoric acid belongs.

This is a godsend for the chemist because it saves him a lot of complicated calculations.

About phosphoric acid:
Given: 0.1 mol / l H3PO4

pKs H3PO4 = 2,12
From this value it follows that phosphoric acid is neither a strong nor a weak one. So how do you calculate the Ph value?

1st possibility: One defines phosphoric acid as a weak acid:
then it arises
ph = (pKs - log c) / 2 (imprecise, but simple)

pH = (2.12 + 1) / 2

pH = 1.56

2nd possibility: You insert the numbers directly into the law of mass action:
(more precise, but more complicated)


Computationally exact values ​​for the special cases of medium-strength acids are obtained by inserting the exact values ​​into the law of mass action. Then solve for c (H3O+) on.

c (H3O+) * c (H2PO4-)
Ks =---------------------------------   
c (H3PO4)

c (H3O+) * c (H3O+) (Conc. Of the dihydrogen phosphate ions is the same as that of the oxonium ions)
7,6*10-3 = ---------------------------
0.1 - c (H.3O+) (Conc. Of the acid is reduced by the dissociated Oxonium ions)

7,6*10-3 * (0.1 - c (H.3O+)) = c2 (H3O+)


c (H3O +)2 = 0.0076 * (0.1 - c (H3O +)) = c (H3O +)2


- c (H3O +)2 - 0.0076 c (H3O +) + 0.00076 = 0


c (H3O +)2 + 0.0076 c (H3O +) - 0.00076 = 0


c (H3O +) = - 0.0076 / 2 + [(1/2 * 0.0076)2 + 0,00076]1/2


c (H3O +) = -0.0076 / 2 + 0.0278 (other solution is excluded.)


c (H3O +) = 0.024


pH = 1.619


This leads to a quadratic equation which, according to c (H3O+) dissolved, results in a concentration of 0.024, thus the pH = 1.619

The formula for calculating the pH value for weak acids also provides useful results here, but because of the mutual influence of the ions in the solution, this theoretically calculated value does not exactly match the theoretically calculated value.

The other dissociation stages of the phosphoric acid play no role for the pH value for the reasons mentioned above.



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