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Understanding pH and Ion Product Constant for Water

Learn about pH, the ion product constant for water (Kw), and how to calculate the concentration of H+ and OH- ions. Explore examples and understand the acidic, neutral, and basic nature of solutions.

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Understanding pH and Ion Product Constant for Water

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  1. What is pH?

  2. Ion Product Constant for Water • H2O(l)  H+(aq) + OH-(aq) • Keq = Kw = [H+] x [OH-] • The ion product constant for water (Kw) is the equilibrium constant (Keq) expression for the self-ionization of water. • Kw = 1 x 10-14 at 298K.

  3. H2O(l)  H+(aq) + OH-(aq) • Keq = Kw = [H+] x [OH-] = 1 x 10-14 • [H+] > [OH-] = acidic • [H+] < [OH-] = basic • [H+] = [OH-] = neutral

  4. Calculations of [H+] or [OH-] • Ex 1: At 298 K the H+ ion concentration of an aqueous solution is 1.0 x 10-5M. What is the OH- ion concentration in the solution? Is the solution acidic, basic, or neutral? Kw = [H+] x [OH-] 1 x 10-14= [1.0 x 10-5] x [OH-] 1.0 x 10-5 1.0 x 10-5 1.0 x 10 -9 = [OH-] [H+] & [OH-] comparison 1.0 x 10-5 > 1.0 x 10-9 0.00001 > 0.000000001 [H+] > [OH-] = solution is acidic

  5. Calculations of [H+] or [OH-] • Ex 2: At 298K the H+ ion concentration of an aqueous solution is 2.5 x 10-6M. What is the OH- ion concentration in the solution? Is the solution acidic, basic, or neutral? Kw = [H+] x [OH-] 1 x 10-14= [2.5 x 10-6] x [OH-] 2.5 x 10-6 2.5 x 10-6 4.0 x 10 -9 = [OH-] [H+] & [OH-] comparison 2.5 x 10-6 > 4.0 x 10-9 0.0000025 > 0.000000004 [H+] > [OH-] = solution is acidic

  6. What is pH? • The hydrogen concentration is cumbersome so Soren Sorensen came up with the pH scale. • pH ranges from 0-14 • pH <7 is acidic • pH = 7 is neutral • pH > 7 is basic pH= -log[H+] ***Always use pH to identify if a solution is acidic, neutral or basic, NOT pOH

  7. What is pH?

  8. We use logarithms to do this • N = 10x, then log N = x • pH = -log[H+] • Ex3: What is the pH of a solution with a concentration of 1.0 x 10-6 hydrogen ions? Then tell if it is acidic, neutral or basic. ***There is a shortcut to this, look at the eponent for the given concentration, and that is your pH or pOH depending on which concentration is given pH = -log[H+] pH = -log[1.0 x 10-6 ] pH = 6.0 pH is acidic

  9. pH = -log[H+] • Ex 4: What is the pH of a solution if the concentration of the hydroxide ions is 9.6 x 10-4 M? Then tell if it is acidic, neutral or basic. Kw = [H+] x [OH-] 1 x 10-14= [H+] x [9.6 x 10-4] 9.6 x 10-4 9.6 x 10-4 1.0 x 10 -11 = [H+] pH = -log[H+] pH = -log[1.0 x 10-11] pH = 11 pH > 7 is basic

  10. pOH = -log[OH-] pH + pOH = 14 • Ex 5: Ammonia gas has a hydroxide-ion concentration of 4.0 x10-3 M. Calculate the pOH and pH at 298 K. Then tell if it is acidic, neutral or basic. pOH = 3 (from the exponent) pH + pOH = 14 pH + 3 = 14 pH = 11 pH > 7 is basic 11 > 7 Ammonia solution is basic

  11. pOH = -log[OH-] or pH + pOH = 14 • Ex 6: A solution contains a hydrogen ion concentration of 3.56 x 10-8 M. Calculate the pH and pOH at 298 K. Then tell if it is acidic, neutral or basic. pH = 8 pH + pOH = 14 8+ pOH = 14 pOH = 6 pH > 7 is basic 8 > 7 solution is basic

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