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Chemistry LibreTexts

Base Strength

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  • Base strength of a species is its ability to accept H+ from another species (see, Brønsted-Lowry theory).  The greater the ability of a species to accept a H+ from another species, the greater its base strength.  Organic chemists customarily compare the strength of bases using the strengths of their conjugate acids, measured as pKa.

    eg: Base strengths of F ¯ and HS ¯


    conjugate acid of  F ¯ = F ¯ + H+ = HF
    conjugate acid of  HS ¯ = HS ¯ + H+ = H2S


    HF is a stronger acid than H2S.  The stronger the acid, the weaker the conjugate base, and vice versa.  Thus, HS ¯ is a stronger base than F ¯.