Potassium acetate

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Adding an acid khổng lồ water increases the H3O+ion concentration & decreases the OH- ionconcentration. Adding a base does the opposite. Regardless ofwhat is added khổng lồ water, however, the sản phẩm 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+& 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 anarrow range of concentrations between 1 x 10-7 Mvà 1 x 10-6 M. The point at which theconcentrations of the H3O+ and OH-ions are equal is called the neutral point. 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 than 1 x 10-7 M are basic.

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It is impossible khổng lồ construct a graph that includes all thedata from the table given above sầu. In 1909, the Danish bioanhhungxadieu.vnistS. P. L. Sorenson proposed using logarithmic mathematics tocondense the range of H3O+ và OH-concentrations khổng lồ a more convenient scale. By definition, thelogarithm of a number is the power lớn which a base must be raisedkhổng lồ obtain that number. The logarithm lớn the base 10 of 10-7for example, is -7.

log (10-7) = -7

Since the concentrations of the H3O+ andOH- ions in aqueous solutions are usually smaller than1 M, the logarithms of these concentrations are negativenumbers. Because he considered positive sầu numbers more convenient,Sorenson suggested that the sign of the logarithm should bechanged after it had been calculated. He therefore introduced thesymbol "p" lớn indicate the negative of thelogarithm of a number. Thus, pH is the negative sầu of thelogarithm of the H3O+ ion concentration.

pH = - log

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

pOH = - log

pH + pOH = 14

The equation above sầu can be used to lớn convert from pH khổng lồ pOH, orvice versa, for any aqueous solution at 25C, regardless of howmuch acid or base has been added to lớn the solution. By convertingthe H3O+ and OH- ionconcentrations in the table above inkhổng lồ pHand pOH data, we can fit the entire range of concentrations ontoa single graph, as shown in the figure below.

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Acid-DissociationEquilibrium Constants

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

By definition, a svào acid is any substance that is good atdonating an H+ ion lớn water.

Example: 99.996% of the HCl molecules in a 6 M solutiondissociate when the following reaction comes khổng lồ equilibrium. Thisequilibrium lies so far to lớn the right that we write the equationfor the reaction with a single arrow, suggesting thathydrochloric acid dissociates more or less completely in aqueoussolution.

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

Weak acids are relatively poor H+ ion donors.

Example: Acetic acid is a Brnsted acid because it can donatean H+ ion lớn 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 chiến bại a proton khổng lồ water.


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

A quantitative sầu feeling for the difference between strong acidsvà weak acids can be obtained from the equilibrium constants forthe reactions between acids and water. Because it istime-consuming khổng lồ write the formula CH3CO2Hfor acetic acid, anhhungxadieu.vnists commonly abbreviate this formula asHOAc & describe the dissociation of the acid as follows.

HOAc(aq) + H2O(l)

Using this convention, the equilibrium constant expression forthe reaction between acetic acid and water would be written asfollows.

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Like the equilibrium constant expression for the dissociationof water, this is a legitimate equation. But most acids are weak,so the equilibrium concentration of H2O is effectivelythe same after dissociation as before the acid was added. Becausethe term has no effect on the equilibrium it isbuilt inlớn the equilibrium constant for the reaction as follows.

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

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In general, for any acid HA:

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Values of Ka can be used khổng lồ estimatethe relative sầu strengths of acids. The larger the value of Ka,the stronger the acid. By definition, a compound is classified asa strong acid when Ka is larger than 1.Weak acids have sầu values of Ka that aresmaller than 1. A các mục of the acid-dissociation equilibriumconstants for some common acids is given in the table below.

Values of Ka forCommon 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 above sầu provides us with the basis for understandingthe difference between strong acids & weak acids. Think aboutthe reaction between a very strong acid và water.


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

HCl is a much stronger acid than the H3O+ion. This means that H2O is a stronger base than theCl- ion. It isn"t surprising to find that the strongerof a pair of acids reacts with the stronger of a pair of bases togive sầu a weaker acid and a weaker base.

Let"s consider the reaction between acetic acid & 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 khổng lồ convert the weaker of apair of acids & the weaker of a pair of bases inkhổng lồ a strongeracid & a stronger base. It isn"t surprising to lớn find that thisreaction occurs khổng lồ only a minor extent.

As the value of Ka decreases further the extent towhich the acid will react with water must decrease as well.Inevitably, we should encounter acids that are so weak they can"tcompete with water as a source of the H3O+ion.


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