Adding an acid to water boosts the H3O+ion concentration and also decreases the OH- ionconcentration. Adding a base does the opposite. Regardless ofwhat is added to water, however, the product of theconcentrations of these ions at equilibrium is always 1.0 x 10-14at 25oC.

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=1.0 x 10-14

The table below lists pairs of H3O+ andOH- ion concentrations that can coexist at equilibriumin water at 25oC.

Pairs of EquilibriumConcentrations of H3O+and also OH- Ions That Can Coexist in Water

Concentration (mol/L)
1 1 x 10-14 " width="27" height="181">
1 x 10-1 1 x 10-13
1 x 10-2 1 x 10-12
1 x 10-3 1 x 10-11 Acidic Solution
1 x 10-4 1 x 10-10
1 x 10-5 1 x 10-9
1 x 10-6 1 x 10-8
1 x 10-7 1 x 10-7 Neutral Solution
1 x 10-8 1 x 10-6 " width="27" height="181">
1 x 10-9 1 x 10-5
1 x 10-10 1 x 10-4
1 x 10-11 1 x 10-3 Basic Solution
1 x 10-12 1 x 10-2
1 x 10-13 1 x 10-1
1 x 10-14 1

Data from this table are plotted in the figure below over anarrowhead variety of concentrations between 1 x 10-7 Mand 1 x 10-6 M. The suggest at which theconcentrations of the H3O+ and OH-ions are equal is referred to as the neutral suggest. Solutions inwhich the concentration of the H3O+ ion islarger than 1 x 10-7 M are described as acidic.Those in which the concentration of the H3O+ion is smaller sized than 1 x 10-7 M are basic.


It is impossible to construct a graph that has all thedata from the table given above. In 1909, the Danish biosoimg.orgistS. P. L. Sorenkid proposed making use of logarithmic math toconthick the selection of H3O+ and also OH-concentrations to a more convenient scale. By meaning, thelogarithm of a number is the power to which a base should be raisedto obtain that number. The logarithm to the base 10 of 10-7for example, is -7.

log (10-7) = -7

Since the concentrations of the H3O+ andOH- ions in aqueous remedies are normally smaller than1 M, the logarithms of these concentrations are negativenumbers. Since he taken into consideration positive numbers even more convenient,Sorenboy said that the sign of the logarithm must bereadjusted after it had been calculated. He therefore introduced thesymbol "p" to show the negative of thelogarithm of a number. Therefore, pH is the negative of thelogarithm of the H3O+ ion concentration.

pH = - log

Similarly, pOH is the negative of the logarithm of theOH- ion concentration.

pOH = - log

pH + pOH = 14

The equation over have the right to be used to transform from pH to pOH, orvice versa, for any kind of aqueous solution at 25C, regardmuch less of howa lot acid or base has been added to the solution. By convertingthe H3O+ and also OH- ionconcentrations in the table over right into pHand also pOH data, we have the right to fit the whole selection of concentrations ontoa solitary graph, as shown in the figure listed below.


Acid-DissociationEquilibrium Constants

Tright here is a big difference between strong acids such ashydrochloric acid and weak acids such as the acetic acid invinegar. Both compounds satisfy the Brnsted interpretation of anacid. (They are both H+ ion, or proton, donors.) Butthey differ in the extent to which they donate H+ ionsto water.

By definition, a solid acid is any kind of substance that is good atdonating an H+ ion to water.

Example: 99.996% of the HCl molecules in a 6 M solutiondissociate as soon as the adhering to reactivity concerns equilibrium. Thisequilibrium lies so much to the appropriate that we create the equationfor the reaction via a solitary arrow, saying thathydrochloric acid dissociates even more or less completely in aqueoussolution.

HCl(aq) + H2O(l)
" width="17" height="9">
H3O+(aq) + Cl-(aq)
0.004% 99.996%
at equilibrium at equilibrium

Weak acids are reasonably negative H+ ion donors.

Example: Acetic acid is a Brnsted acid bereason it deserve to donatean H+ ion to water. But it isn"t a very good H+ion donor. Only about 1.3% of the acetic acid molecules in an0.10 M solution lose a proton to water.

CH3CO2H(aq) + H2O(l ) H3O+(aq) + CH3CO2-(aq)
98.7% 1.3%
at equilibrium at equilibrium

A quantitative feeling for the distinction in between strong acidsand also weak acids deserve to be derived from the equilibrium constants forthe reactions in between acids and also water. Since it istime-consuming to create the formula CH3CO2Hfor acetic acid, soimg.orgists frequently abbreviate this formula asHOAc and describe the dissociation of the acid as complies with.

HOAc(aq) + H2O(l)

Using this convention, the equilibrium consistent expression forthe reaction between acetic acid and also water would certainly be composed ascomplies with.


Like the equilibrium constant expression for the dissociationof water, this is a legitimate equation. But a lot of acids are weak,so the equilibrium concentration of H2O is effectivelythe very same after dissociation as before the acid was added. Becausethe term has actually no result on the equilibrium it isdeveloped into the equilibrium constant for the reactivity as adheres to.


The result is an equilibrium continuous for this equation knownas the acid-dissociation equilibrium constant, Ka.For this reaction:


In general, for any type of acid HA:


Values of Ka have the right to be offered to estimatethe loved one staminas of acids. The larger the worth of Ka,the more powerful the acid. By meaning, a compound is classified asa solid acid once Ka is bigger than 1.Weak acids have worths of Ka that aresmaller than 1. A list of the acid-dissociation equilibriumconstants for some prevalent acids is provided in the table below.

Values of Ka forUsual Acids

Strong Acids Ka
hydrochloric acid (HCl) 1 x 106
sulfuric acid (H2SO4) 1 x 103
hydronium ion (H3O+) 55
nitric acid (HNO3) 28
Weak Acids Ka
phosphoric acid (H3PO4) 7.1 x 10-3
citric acid (C6H7O8) 7.5 x 10-4
acetic acid (CH3CO2H) 1.8 x 10-5
boric acid (H3BO3) 7.3 x 10-10
water (H2O) 1.8 x 10-16

The table over gives us via the basis for understandingthe distinction between strong acids and also weak acids. Think aboutthe reaction in between an extremely solid acid and also water.

HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq)
Ka = 106 Ka = 55

HCl is a much stronger acid than the H3O+ion. This implies that H2O is a more powerful base than theCl- ion. It isn"t surpclimbing to uncover that the strongerof a pair of acids reacts through the more powerful of a pair of bases toprovide a weaker acid and a weaker base.

Let"s think about the reactivity in between acetic acid and also water.

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HOAc(aq) + H2O(l) H3O+(aq) + OAc-(aq)
Ka = 1.8 x 10-5 Ka = 55

In this case, the reaction tries to convert the weaker of apair of acids and the weaker of a pair of bases into a strongeracid and a stronger base. It isn"t surprising to discover that thisreaction occurs to just a minor extent.

As the value of Ka decreases further the extent towhich the acid will react through water must decrease too.Inevitably, we have to enrespond to acids that are so weak they can"tcomplete via water as a source of the H3O+ion.