Equilibrium of Water Pure water contains very small concentrations of hydrogen, H+, and hydroxide, OH-, ions. While the “H+” actually pairs with a water.

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Presentation transcript:

Equilibrium of Water Pure water contains very small concentrations of hydrogen, H+, and hydroxide, OH-, ions. While the “H+” actually pairs with a water to become hydronium, H3O+, we write H+ for convenience. In a sample of pure water at 25°C the concentration of hydrogen and hydroxide ions is 0.0000001 moles/L or 1.0 x 10-7 M each. What is the Keq, then?

Keq for Water… or Kw So Keq or Kw = [H+][OH-] Kw = (1.0 x 10-7 M) x (1.0x10-7 M) = 1.0 x 10-14 No matter how acidic or basic a solution is, it contains both H+ and OH- ions- they are in equilibrium. The product of the concentrations must always equal 1.0 x 10-14 When one goes up the other must go down proportionally. If [H+] was 1.0 x 10-9 M what would [OH-] have to be?

Creating the pH, pOH, pKw system Kw = (1.0 x 10-7 M) x (1.0x10-7 M) = 1.0 x 10-14 So mathematically: 7 + 7 = 14… an easier system to consider. If we remove the negative exponential form, the numbers are simpler… and Chemists are Lazy. Logarithms pull numbers out of exponential form Take the negative logarithm of everything and call it ‘p’ pH + pOH = pKw or 14 So, again… if [H+] was 1.0 x 10-9 M what would [OH-] have to be?

pH is a measure of the concentration of the H+ ion. pH = -log [H+] The hydrogen ion concentration [H+] of a solution is 1 x 10-10 M. What is the pH? pH = -log [1 x 10-10] pH = 10 pOH? [OH-]?

pH is a measure of the concentration of the H+ ion. The [H+] concentration of a solution is 7.3 x 10-5 M. What is the pH? pH = -log (7.3 x 10-5) pH = 4.1 Is it an Acid or a Base? This is an acid.

pH is a measure of the concentration of the H+ ion. [H+] = 10 -pH The pH of a solution is 5. What is the hydrogen ion concentration [H+]? [H+] = 1.0 x 10-5 or 0.00001 M pOH? [OH-]?

pH is a measure of the concentration of the H+ ion. The pH of a solution is 8.2. What is the [H+] concentration? [H+] = 1.0 x 10-8.2 [H+] = 6.3 x 10-9 or 0.0000000063 M Is it an Acid or a Base? This is a base.