PH Scale.

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Presentation transcript:

pH Scale

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

If the pH changes from 5 to 7, how do the [H+] compare? Difference in pH = 2, which means the [H+] differ by 102 or 100. The pH of 5 solution has an [H+] that is 100 X greater than that of the pH 7 solution. If the pH changes from 5 to 7, how do the [H+] compare?

Molarity to pH To determine pH: Express [H+] in scientific notation. Remember, [ ] means concentration of whatever is inside the brackets. The log is the power of 10.

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

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

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

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

pH + pOH = 14 pH = 3, pOH = 11 pH = 7, pOH = pH = 10, pOH = 7 5 13

ACID or BASE? Acids, [H+]  [OH-] Bases, [OH-]  [H+]

0 is most acidic; 14 is most basic

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

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

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

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

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

How to safely test pH Instrumental – use a pH meter Indicators – use a series of indicators See if the substance reacts with a metal other than Cu, Ag, or Au. Do NOT say “taste.”

Indicator A substance that changes color over a narrow pH range. Use several indicators to narrow down the pH range of the substance.