pH, pKa, Ka, pKb, Kb

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Ranking Acid Base Strength Using Ka pKa Values Leah4sci

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Key Questions

  • The pH scale provides a way of measuring how acidic or basic solutions are. The scale ranges from 0-14. A pH of 0 is the most acidic, 7 is neutral and 14 is the most basic.

    Here is a video of a lab which looks at a number of different solutions and measures their pH levels using a pH meter and an indicator.

    video from: Noel Pauller

  • #"p"K_"a"# and #"p"K_"b"# are measures of the strengths of acids and bases, respectively

    Acids

    When you dissolve an acid in water, it undergoes an equilibrium reaction with the water in an.

    HA + H₂O ⇌ H₃O⁺ + A⁻

    The value of the equilibrium constant is given by

    #K_"a" = (["H"_3"O"^+]["A"^-]]/["HA"]#

    The greater the value of #K_"a"#, the stronger the acid.

    For most weak acids, #K_"a"# ranges from #10^-2# to #10^-14#.

    We convert these exponential numbers into a normal range by taking their negative logarithm.

    The operator #"p"# means "take the negative logarithm of".

    So #"p"K_"a" = -logK_"a"#.

    For most weak acids, #"p"K_"a"# ranges from 2 to 14.

    Thus, the smaller the value of #"p"K_"a"# , the stronger the acid.

    Bases

    When you dissolve a base in water, it reacts with the water in an equilibrium reaction.

    B + H₂O ⇌ BH⁺ + OH⁻

    The value of the equilibrium constant is given by

    #K_"b" = (["BH"^+]["OH"^-]]/["B"]#

    The greater the value of #K_"b"#, the stronger the base.

    For most weak acids, #K_"b"# ranges from #10^-2# to #10^-13#.

    #"p"K_"b" = -logK_"b"#.

    For most weak acids, #"p"K_"a"# ranges from 2 to 13.

    The smaller the value of #"p"K_"b"# , the stronger the base.

    Here's a video on #"p"K_"a"# and #"p"K_"b"#.

  • Answer:

    Ka= dissociation constant of acid; Kb= dissociation constant of base

    Explanation:

    this concept of Ka and Kb is valid only for weak acids and weak bases.
    let HA be a weak acid, whose dissociation reaction is:

    #HA_(aq) rarr H+_(aq) + A-(aq)#
    This is a reversible reaction

    then Ka = [H+][A-] / [HA]
    here A- is conjugate base, writing above reaction in reverse order

    #A-_(aq) rarr HA + -OH#

    Kb for this reaction is:
    #Kb = ([HA][-OH])/[A-]#

    Note: lower is the pKa value stronger is the acid
    higher is the pKb value stronger is the base.

Questions

  • anor277 answered · 7 months ago