Basic chemistry

Ka to pka ?

Ka to pka

Ka to pka?

What is Ke and Ka? Be careful not to confuse Ke and Ka. Indeed, Ke is a constant related to the autoprotolysis of water.

The Ka corresponds to the acidity constant, also called acid dissociation constant. It allows the quantitative measurement of the strength of an acid in solution. Indeed, this constant also makes it possible to know the equilibrium of an acid species in the context of an acid-base reaction. In this way, the higher the Ka, the greater the dissociation of the molecules in solution. We can then say that the strength of an acid is proportional to Ka.

In chemistry, an acid constant or acid dissociation constant, Ka, is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant of the dissociation reaction of an acidic species in the context of acid-base reactions. The higher this Ka constant, the greater the dissociation of the molecules in solution, and therefore the stronger the acid.

Is :

  \ [AH + B \ rightleftarrows A ^ {-} + BH ^ {+} \]


  • Couple AH / A 
  • Couple BH + / B

It is then considered that AH corresponds to a generic acid which dissociates to give the acid A  which is its conjugate base, as well as a hydrogen ion H + which is also called proton. If the reaction takes place in an aqueous medium, this proton becomes a solvated proton H 3 O + .

The chemical species AH, A  and H + are then considered to be in equilibrium if their concentration does not vary with time. One can then write the equilibrium constant in the form of concentration quotients of the different species at equilibrium (in mol / L), denoted by [AH], [A  ] and [H + ]. We thus obtain:

  \ [K _ {a} = \ frac {\ left [A ^ + \ right] \ times \ left [H ^ - \ right]} {\ left [AH \ right]} \]

To obtain the pKa, just like the pH value, the p of pKa represents the “- log” function, hence here – log (Ka). So if we talk about pOH or pKa, it means that we talk about – log (OH) and – log (Ka). So we have :

  \ [\ text {p} K _ {a} = \ log \ left (10 \ right) \ times K _ {a} \]

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