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Chapter 16: Equilibrium in Acid-Base Systems

Chapter 16: Equilibrium in Acid-Base Systems. 16.1a: Self-ionization of Water K w pH and pOH. Self-ionization of water. If water is a weak electrolyte, what does it ionize in to? H 2 O (l) ⇄ OH - ( aq ) + H + ( aq ) OR 2H 2 O (l) ⇄ OH - ( aq ) + H 3 O + ( aq )

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Chapter 16: Equilibrium in Acid-Base Systems

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  1. Chapter 16: Equilibrium in Acid-Base Systems 16.1a: Self-ionization of Water Kw pH and pOH

  2. Self-ionization of water • If water is a weak electrolyte, what does it ionize in to? H2O (l) ⇄ OH- (aq) + H+ (aq) OR 2H2O (l) ⇄ OH- (aq) + H3O+ (aq) • the concentrations of the ions are only 1.0 x 10-7 M at 25°C • the product of these concentrations will remain constant for any solution at the same temperature

  3. Self-ionization of Water • concentrations or molarities can be written with brackets • For example: concentration of A = [A] = 2.0 M • Kw: • the ionization constant of water • the product of [OH-] and [H+] • at 25oC

  4. Kw • subject to the same restricition as any other equilibrium constant (T, P) • Will acidic solutions have more H+ or OH-? • [H+]>[OH-]: acidic • [OH-]>[H+]: basic • [OH-]=[H+]: neutral • can find the [OH-] or [H+] from a mole ratio of the dissociation or reaction in the water of the acid or base

  5. Example 1 • Calculate the H+ concentration in a 1.5 M Ca(OH)2 solution • What does the Ca(OH)2 create in solution? • Ca(OH)2 Ca2++ 2OH- • can calculate the OH- concentration • use Kw to calculate the H+ concentration

  6. Example 2 • Calculate the H+ and OH- concentration of a 1.0x10-4 M solution of HNO3 • Find the [H+] from mole ratio • Find [OH-] from Kw and [H+] HNO3 H+ + NO3-

  7. pH scale • more convenient than using concentrations • pH=-log [H+] • pOH=-log [OH-] • pH increases as [H+] decreases • pH < 7: acid • pH > 7: base • pH = 7: neutral

  8. pH • for any solution at 25oC: • -log[H+] + -log[OH-] = -log([H+][OH-] • = -log(1.0x10-14) =14 • 14.00 = pH + pOH

  9. Example 3 • Find the pH of a 1.0x10-3 M NaOH solution • find [OH-] using mole ratio • find [H+] from Kw • find pH from [H+] OR • find pOH from [OH-] • find pH from pOH

  10. Example 3 sol’n OR

  11. Calculating [H+] and [OH-] • reverse the pH equation • The pH of a solution is 7.52. Find the [H+] and [OH-] and determine whether it is acidic, basic, or neutral. [OH-] > [H+] so solution is basic

  12. Example • A shampoo has a pH of 2.53. Calculate the pOH, [H+] and [OH-]. Is it acidic, basic, or neutral? pH < 7 so acidic

  13. Homework • Textbook p716 #1-6 • p718 #7 • LSM 16.1B

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