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pH scale

pH scale. Logarithmic scale expresses the H +1 concentration [H +1 ] If the pH changes by a factor of 1: the [H +1 ] changes by a factor of 10 pH = -log[H +1 ]. Molarity & pH. To determine pH: Express [H +1 ] in scientific notation

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pH scale

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  1. pH scale • Logarithmic scale • expresses the H+1 concentration [H+1] • If the pH changes by a factor of 1: • the [H+1] changes by a factor of 10 • pH = -log[H+1]

  2. Molarity & pH • To determine pH: • Express [H+1] in scientific notation • [ ] means concentration of whatever is inside the brackets • The log is the power of 10

  3. Molarity & pH for Pure water • a tiny bit of ionization: H2O  H+ + OH- • [H+1] = [OH-1] = 1 X 10-7M • pH =-log[H+1]= -(-7) = 7

  4. [H+] 1 M or 1X100 M 0.1 M or 1 X 10-1M 0.01 M or 1 X 10-2M .001 M or 1 X 10-3M pH 0 1 2 3 pH

  5. pH to Molarity • pH = -log[H+1] • Solve for [H+1] • -pH = log[H+1] • Antilog(-pH) = [H+1] • Say pH = 5; then –pH = -5 • Antilog(-5) = 10-5 • The (–) pH becomes the power of 10!

  6. pOH • By analogy, pOH is defined as –log[OH-1] • Express [OH-1] in scientific notation • If [OH-1] = 1.0 X 10-3 M, then pOH = -log(10-3) = -(-3) = 3

  7. 11 4 5 13 pH + pOH = 14 • pH = 3, pOH = • pH = 7, pOH = • pH = 10, pOH = • pH = 9, pOH = • pH = 1, pOH = 7

  8. ACID or BASE? • Acid: [H+1]  [OH-1] • Base: [OH-1]  [H+1]

  9. pH range • 0 to 14 • 7 is neutral • pH = 0, strongly acidic • pH = 14, strongly basic

  10. Antilog(-pH) = [H+1] Antilog(-4) = 10-4 10 Antilog(-10) = [OH-1] 10-10 = [OH-1] If the pH = 4 • [H+1] = ? • pOH = ? • [OH-1] = ?

  11. 14 – 3 = 11 Antilog(-11) = 10-11 M If the [OH-1] = 1 X 10-3 pOH = -log[OH-1] = -log(10-3) = -(-3) = 3 • pOH = ? • pH = ? • [H+1] = ?

  12. 5 14 – 5 = 9 Antilog(-9) = 10-9 If the[H+1] = 1 X 10-5 M • pH = ? • pOH = ? • [OH-1] = ?

  13. [H+1] pH [OH-1] pOH Acidic or Basic 3 10-3 A 10-11 11 10-5 M 9 5 B 10-9 10-2 M A 12 2 10-12 M 6 B 10-6 M 10-8 M 8

  14. How to safely test pH • Instruments – use a pH meter • Indicators – use a series of indicators • See if substance reacts with a metal other than Cu, Ag, or Au (think about table J) • NEVER “taste”

  15. Indicator • substance that changes color over a narrow pH range • can use several indicators to narrow the pH range of the substance

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