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The Autoionization of Water

H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH – (aq). [H + ] > [OH – ] . [H + ] < [OH – ] . [H + ] = [OH – ] . The Autoionization of Water. In water, we constantly have (a little). Thus (@ 25 o C), the eq. const. for water is:.

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The Autoionization of Water

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  1. H2O(l) + H2O(l) H3O+(aq) + OH–(aq) [H+] > [OH–] [H+] < [OH–] [H+] = [OH–] The Autoionization of Water In water, we constantly have (a little)... Thus (@ 25oC), the eq. const. for water is: Kw = [H3O+] [OH–] = [H+] [OH–] = 1.0 x 10–14 This equation is taken to be valid for pure water and for dilute aqueous solutions, @ 25oC. ACID BASE NEUTRAL

  2. At 25oC, calculate the hydrogen ion concentration if the hydroxide ion concentration is 2.7 x 10–4 M. Is this solution an acid or a base? [H+] [OH–] = 1.0 x 10–14 [H+] (2.7 x 10–4) = 1.0 x 10–14 [H+] = 3.7 x 10–11 M [H+] < [OH–] base

  3. as [H+] / , pH / : Soren Sorensen (1868–1939) The pH Scale Definition of pH... pH = –log [H3O+] = –log [H+] -- 0 ACID 7 BASE 14 neutral A change of 1 pH unit means that [H+] (and [OH–]) has changed by a factor of ___. 10 A solution with pH 2 is how many times more acidic than one with pH 5? 1000X more acidic (i.e., 1000X less basic)

  4. Especially in biochemical reactions, the rate law is often dependent on the hydrogen ion concentration. If the rate is 1st order in [H+], then doubling [H+] would double the rate. -- Human blood pH is between 7.35 and 7.45. red blood cell white blood cell platelet Scanning electron microscope micrograph of ordinary blood.

  5. 3.9 x 10–10 ** Significant figures rule for logarithms: # of sig. figs. in [ ] = # of decimal places in pH Find the pH if the hydronium ion concentration is 5.2 x 10–9 M. pH = –log [H3O+] = –log (5.2 x 10–9) = 8.28 Find the pH if the hydroxide ion concentration is 3.9 x 10–10 M. [H+] [OH–] = 1.0 x 10–14 Blueberries grow best in soil having a pH between 4.5 and 5.0. So [H+] = 2.56 x 10–5 M pH = –log [H+] = –log (2.56 x 10–5) = 4.59

  6. Other equations: [OH–] = 10–pOH pOH = –log [OH–] [H+] = 10–pH pH + pOH = 14 Find [H+] if the pH = 5.22. [H+] = 10–pH = 10–5.22 = 6.0 x 10–6 M

  7. a pair of electrodes Bromthymol blue is yellow at pH < 6.0 and blue at pH > 7.6. Measuring pH pH meter acid-base indicators: chemicals whose color depends on pH e.g., litmus, phenolphthalein, bromthymol blue, methyl red, methyl orange pH paper is paper impregnated with mixtures of various indicators.

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