Key PointsAn acid dissociation continuous (Ka) is a quantitative measure of the strength of an mountain in solution.The dissociation consistent is normally written together a quotient of the equilibrium concentration (in mol/L): K_a = frac .Often times, the Ka value is to express by using the pKa, which is equal to -log_10(K_a) . The larger the worth of pKa, the smaller the level of dissociation.A weak acid has actually a pKa worth in the approximate selection of -2 come 12 in water. Acids v a pKa value of less than around -2 are stated to be strong acids.

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TermsequilibriumThe state the a reaction in i m sorry the rates of the forward and also reverse reactions are equal.dissociationReferring come the process by which a compound breaks right into its constituent ion in solution.

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

HA(aq) ightleftharpoons H^+(aq) + A^-(aq)

In the over reaction, HA (the share acid), A– (the conjugate basic of the acid), and H+ (the hydrogen ion or proton) are claimed to be in equilibrium when their concentrations execute not adjust over time. Just like all equilibrium constants, the value of Ka is established by the concentration (in mol/L) of every aqueous species at equilibrium. The Ka expression is as follows:

K_a=frac

Acid dissociation constants are many often connected with weak acids, or mountain that carry out not completely dissociate in solution. This is because strong acids space presumed to ionize totally in solution and also therefore your Ka values space exceedingly large.

Ka and pKa

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

The bigger the value of pKa, the smaller the degree of dissociation. A weak acid has actually a pKa value in the approximate range of -2 to 12 in water. Acids with a pKa value of less than around -2 are claimed to be strong acids. A solid acid is almost totally dissociated in aqueous solution; that is dissociated come the extent that the concentration that the undissociated mountain becomes undetectable. PKa worths for strong acids can be estimated by theoretical method or by extrapolating from measurements in non-aqueous solvents with a smaller dissociation constant, such together acetonitrile and also dimethylsulfoxide.

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

Example

Acetic mountain is a weak acid through an mountain dissociation continuous K_a=1.8 imes 10^-5 . What is the pKa for acetic acid?

pK_a=-log(1.8 imes 10^-5)=4.74


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