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Chapter 19 Notes, part III

Chapter 19 Notes, part III. pH and [H + ]. Hydrogen and hydroxide in H 2 O. Working with the Arrhenius acid definition, we say that acids are: HX a H + + X - And that a base would be: XOH a X + and OH -. Hydrogen and hydroxide in H 2 O.

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Chapter 19 Notes, part III

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  1. Chapter 19 Notes, part III pH and [H+]

  2. Hydrogen and hydroxide in H2O • Working with the Arrhenius acid definition, we say that acids are: • HX a H+ + X- • And that a base would be: • XOH a X+ and OH-

  3. Hydrogen and hydroxide in H2O • Water, being made up of both H+ and OH- can dissociate giving us the formula: • H2O a H+ + OH- • But it only does once out of every 500,000,000 molecules!

  4. Hydrogen and hydroxide in H2O • We can measure how much H+ and OH- there is in regular water. • [H+]=1x10-7M and [OH-]=1x10-7M • From this information, we can get a dissociation constant for water.

  5. Hydrogen and hydroxide in H2O • Kw=[H+]x[OH-] • Kw=(1.0x10-7M)x(1.0x10-7M) • Kw= 1x10-14

  6. Acids, Bases and Concentration • Even if a solution is acidic or basic, [H+][OH-] = 1x10-14 • Like a see-saw, raising or lowering the amount of H+ will change the amount of OH-.

  7. The pH Scale • The pH scale is a logarithmic scale from 0 to 14. It measures the concentration of H+ (pH stands for “potential hydrogen”) with the formula: pH = -log[H+]

  8. The pH Scale • Water has a pH of 7.0, which is neutral. • Less than 7.0 is acidic (the closer to zero the stronger the acid). • More than 7.0 is basic (the closer to 14 the stronger the base).

  9. Obj. 11-12…pH and pOH Scales • pH = ‘power of hydrogen ion’ • pH scale… • 0 -14 • 7 = neutral • 0 - 7 = acidic • 7 - 14 = basic/alkaline • 0 = strongest acid • 14 = strongest base • logarithmic scale…each step is ten times Stronger/weaker than the one next to it!

  10. The pH Scale • Another way to look at it is the concentration of hydrogen in a solution is 1x10-xM. X is the pH.

  11. The pOH Scale • The pOH scale measures the hydroxide ion concentration. • Because Kw=[H+][OH-]=1x10-14, 14=pH + pOH • Given this, you can solve for pH, pOH and [OH-] given just [H+].

  12. Practice Problem #1 • What is the pH of a solution where [H+] = 1.0x10-3M?

  13. Practice Problem #2 • What is the hydrogen ion concentration of a solution with a pH of 9.6?

  14. Practice Problem #3 • What is the pOH of a solution with a pH of 6.2?

  15. Practice Problem #4 • What is the pOH of a solution with a [H+]=2.9x10-11M?

  16. Practice Problem #5 • Find [OH-], pH and pOH of a solution where [H+]=7.2x10-2M.

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