Key PointsAn acid dissociation consistent (Ka) is a quantitative measure of the stamin of an mountain in solution.The dissociation consistent is generally written as a quotient the the equilibrium concentrations (in mol/L): K_a = \frac .Often times, the Ka worth is express by making use of the pKa, i beg your pardon is same to -log_10(K_a) . The bigger the value of pKa, the smaller the extent of dissociation.A weak acid has a pKa value in the approximate range of -2 to 12 in water. Acids with a pKa worth of much less than about -2 are stated to be solid acids.

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TermsequilibriumThe state of a reaction in i m sorry the prices of the forward and reverse reactions are equal.dissociationReferring to the procedure by i m sorry a compound breaks right into its constituent ion in solution.

The mountain dissociation continuous (Ka) is a quantitative measure up of the toughness of an acid in solution. Ka is the equilibrium constant for the adhering to dissociation reaction of an acid in aqueous solution:

HA(aq) \rightleftharpoons H^+(aq) + A^-(aq)

In the above reaction, HA (the share acid), A– (the conjugate base of the acid), and also H+ (the hydrogen ion or proton) are said to it is in in equilibrium as soon as their concentrations carry out not adjust over time. Just like all equilibrium constants, the value of Ka is figured out by the concentrations (in mol/L) of every aqueous varieties at equilibrium. The Ka expression is as follows:

K_a=\frac

Acid dissociation constants are most often connected with weak acids, or mountain that do not fully dissociate in solution. This is because strong acids space presumed come ionize completely in solution and also therefore their Ka values space exceedingly large.

Ka and pKa

Due come the many orders the magnitude covered by Ka values, a logarithmic measure up of the acid dissociation constant is much more commonly offered in practice. The logarithmic consistent (pKa) is equal to -log10(Ka).

The larger the value of pKa, the smaller the extent of dissociation. A weak acid has actually a pKa worth in the approximate range of -2 to 12 in water. Acids with a pKa worth of much less than about -2 are said to be strong acids. A strong acid is almost totally dissociated in aqueous solution; the is dissociated to the degree that the concentration that the undissociated mountain becomes undetectable. PKa values for strong acids have the right to be approximated by theoretical way or through extrapolating from measurements in non-aqueous solvents with a smaller dissociation constant, such as acetonitrile and dimethylsulfoxide.

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Acetic mountain dissociationThe acetic mountain partially and also reversibly dissociates into acetate and hydrogen ions.

Example

Acetic acid is a weak acid v an mountain dissociation constant K_a=1.8\times 10^-5 . What is the pKa for acetic acid?

pK_a=-log(1.8\times 10^-5)=4.74


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