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Strengths of Acids & Bases Section 20.4

Strengths of Acids & Bases Section 20.4. Strength of an acid or base depends on the degree to which they ionize (break down) in water. Strong Acids: completely ionized in aqueous solution Ex. HCl & H 2 SO 4 HCl + H 2 O --> H 3 O + + Cl -.

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Strengths of Acids & Bases Section 20.4

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  1. Strengths of Acids & BasesSection 20.4 • Strength of an acid or base depends on the degree to which they ionize (break down) in water Strong Acids: completely ionized in aqueous solution Ex. HCl & H2SO4 HCl + H2O --> H3O+ + Cl- Table 20.7-- Strengths of Common Acids & Bases

  2. Weak Acids: ionize only slightly in aqueous solution Ex. Acetic acid CH3COOH + H2O  H3O+ + CH3COO- Table 20.7-- Strengths of Common Acids & Bases

  3. Acid-dissociation Constant • Ratio of the dissociated (ionized) form of an acid to the concentration of the nonionized form • Ka = [H+] [CH3COO-] [CH3COOH] If Ka is small, degree of ionization if small = weak acid If Ka is large, ionization of acid is more complete = strong acid

  4. Strong Bases: dissociate completely into metal ions and hydroxide ions in aqueous solution • Ex. Ca(OH)2, Mg(OH)2 • Weak Bases: react with H2O to form OH- and the conjugate acid of the base • NH3 + H2O  NH4+ + OH-

  5. Base Dissociation Constant • Ratio of concentration of conjugate acid times the concentration of the hydroxide ion to the concentration of the conjugate base. • Kb = [NH4+] [OH-] [NH3] If Kb is large = stronger base ◊◊◊ If Kb is small = weak base

  6. A 0.18M solution of monoproticchloroacetic acid (CH2ClCOOH) has a pH of 1.80. Calculate the acid dissociation constant for this acid. Is this a strong or weak acid? Example Problem

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