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IV. Measuring Acid and Base Strength

IV. Measuring Acid and Base Strength. A. Ionization of Water. 1. Definitions. Pure water does not all exist as H 2 O. A very small amount of water breaks apart. H 2 O (l) --> H + (aq) + OH - (aq). This means that in pure water, there are some H +.

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IV. Measuring Acid and Base Strength

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  1. IV. Measuring Acid and Base Strength A. Ionization of Water 1. Definitions Pure water does not all exist as H2O. A very small amount of water breaks apart. H2O(l) --> H+ (aq) + OH- (aq) This means that in pure water, there are some H+ In pure water, the concentration of H+ = 0.0000001 moles per liter This is means [H+] = 1 x 10-7 M So why isn’t water acidic? H+ H+ H+ OH- OH- As each H2O breaks apart, an OH- is made for each H+ OH- The base ion cancels out the acid ion, so water is neutral So in pure water, the [H+] = 1 x 10-7 M and the [OH-] = 1 x 10-7 M Kw = Ionization constant for water Kw = [H+] x [OH-] = (1 x 10-7 M) x (1 x 10-7 M) Kw = 1 x 10-14

  2. 14 12 10 8 6 4 2 0 [H+] [OH-] [H]x [OH-] 14 14 12 12 10 10 8 8 6 6 4 4 2 2 0 [H+] [OH-] [H]x 0 [OH-] [H+] [OH-] [H]x [OH-] 2. Changes in H+ concentration Adding an acid or a base to water will alter the H+ and the OH- concentration Acid Increases [H+] and decreases [OH-] Base Decreases [H+] and increases [OH-] Kw will never change It is always 1 x 10-14 [H+] x [OH-] = 1 x 10-14 Example - If the [H+] in a solution is 1 x 10-6 M, what is the [OH-]? [H+] x [OH-] = 1 x 10-14 1 x 10-6 M x [OH-] = 1 x 10-14 [OH-] = 1 x 10-8M Hint - The powers of the two ions always adds up to -14

  3. Examples 1. When HCl is added to pure water, the [H+] increases to 1 x 10-4M. What is the [OH-]? [OH-] = 1 x 10-10 M 2. What is the [H+] of a basic solution with an [OH-] of 1 x 10-3 M? [H+] = 1 x 10-11 M 3. What is the [H+] and the [OH-] of a 0.10 M solution of HCl? (Assume all of the HCl ionizes) Since all of the HCl ionizes, we should have a [H+] = 0.10 M 0.10 M = 1 x 10-1 M So [H+] = 1 x 10-1 M and [OH-] = 1 x 10-13 M

  4. B. pH 1. Definition Simple way to indicate the acidity or basicity (alkalinity) of a solution pH is defined as the -logarithum of the H+ in solution pH = -log [H+] 6 6 6 6 This means that if the solution has 1 x 10-6 M H+, the pH is 6 -6 Determine the pH of the following solutions [H+] = 1 x 10 -4 M pH = 4 pH = [H+] = 1 x 10 -3 M 3 [H+] = 1 x 10 -9 M pH = 9 2. pH Scale Pure water has [H+] = 1 x 10-7 M, so neutral solutions have a pH of 7

  5. 7 pH scale is based on a 1-14 scale with 7 being neutral 14 0 Basic Acidic If the [H+] is high (1 x 10-3 M), the pH is lower pH = 3 If the [OH+] is high (1 x 10-4 M), the pH is higher pH = 10 Low pH = Acidic Basic High pH = Basis for litmus paper and other acid/base indicators d. pOH Used very little Similar to pH pOH = -log [OH-] 6 6 6 6 If [OH-] = 1 x 10-6 M, then pOH = 6 -6

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