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Acid Strength:

Acid strength is an acid's ability or tendency to lose a proton. Different acids should have different strengths. There are very few strong acids which are completely ionized in water or solution by loosing one proton. Strong acids are strong electrolytes. In other words, one mole of strong acid HA dissolves completely in water, yielding one mole of H+ ion and one mole of conjugate base A-. A strong acid does not have any non ionized acid of HA.

Determining pka and Acid Strength:

Ka is the acid dissociation constant and is also known as acidity constant or acid ionization constant. An acid dissociation constant is a quantitative measure of acid strength in a solution. It is the equilibrium constant for the acid base chemical reactions. 
The equilibrium constant is a characteristic for a reaction.

 

HA (aq) →→ H+ (aq) + A- (aq)
 

Ka, the acid ionization constant is mainly used to measure the propensity of a compound to donate a proton.

Ka = [H+][A−] / [HA]

More commonly a logarithmic measure of the acid dissociation constant is used in practice, due to many order of magnitude spanned by Ka value. Dissociation constant, denoted by pKa, is equal to –log10 Ka (negative log of dissociation constant). Some times it is referred to as an acid dissociation constant, which is a measure of the strength of an acid/base. The most convenient scale of acidity is pKa.

pKa = - log10ka

Generally a more positive value of pKa corresponds to a weaker acid and more negative value of pKa corresponds to a strong acid. So the low value of pKa indicates a stronger acid and higher value of pKa indicates a weaker acid. A strong acid has a pKa value of about less then -1.74.

Basic Strength: Base strength of a species is its ability to accept H+ from another species. The greater the ability of a species to accept a H+ from another species, the greater its base strength. 

For acids: the stronger the acid, the smaller the pKa

For bases: the stronger the base, the larger the pKa

Points to Remember:

  • The greater is the value of Ka, the stronger will be the acid and the weaker will be the base.

  • The greater is the value of pKa, the weaker will be the acid and the stronger will be the base.

  • The greater is the value of pH, the weaker will be the acid and the stronger will be the base. For acids, pH<7pH<7 and for bases, pH>7pH>7.

  • The greater is the value of Kb, the stronger will be the base and the weaker will be the acid.

  • The greater is the value of pKb, the weaker will be the base and the stronger will be the acid.

  • The greater is the value of pOH, the weaker will be the base and the stronger will be the acid. For bases, pOH<7pOH<7 and for acids, pOH>7pOH>7.

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