Topic 08 – Acids/Bases 8.4 – pH Scale

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Presentation transcript:

Topic 08 – Acids/Bases 8.4 – pH Scale

8.4 The pH scale - 1 hour 8.4.1 Distinguish between aqueous solutions that are acidic, neutral or alkaline using the pH scale. (2) 8.4.2 Identify which of two or more aqueous solutions is more acidic or alkaline using pH values. (2) 8.4.3 State that each change of one pH unit represents a 10-fold change in the hydrogen ion concentration [H+(aq)]. (1) 8.4.4 Deduce changes in [H+(aq)] when the pH of a solution changes by more than one pH unit. (3)

8.4 – pH Scale is Log10 based As scientists we don’t like really big or really small numbers. So we use scientific notation. When that gets a bit tiresome, we go even further. Sorensen proposed the pH scale to simplify the comparison of concentrations of H+ ions in solution The pH scale is base on the log10 scale Review: if Log28 = x, then x would equal 3 Log10100 = 2 So the base 10 logarithm scale is simply an exponential scale (scientific notation) written in a different form

8.4 – pH Scale When dealing with acids and bases in chemistry we often deal with very small concentrations of [H+] and [OH-] ions that range from 1x100 to 1x10-14 Instead of using these small values, we break it down into the log10 scale from 0-14 If [H+] = 1x100 M, then Log101 = 0 If [H+] = 1x10-1, then Log100.1 = -1 If [H+] = 1x10-2, then Log100.01 = -2 If [H+] = 1x10-3, then Log100.001 = -3 Instead of log[H+] we use the NEGATIVE -log[H+] = pH BUT, we don’t want negative values, so we simplify

8.4 – pH Scale Find pH from H+ pH = -log[H+] Find H+ from pH [H+] ions in scientific notation [H+] ions traditionally 1x100 1.0 1 1x10-1 0.1 2 1x10-2 0.01 3 1x10-3 0.001 4 1x10-4 0.0001 5 1x10-5 0.00001 6 1x10-6 0.000001 7 1x10-7 0.0000001 8 1x10-8 0.00000001 9 1x10-9 0.000000001 10 1x10-10 0.0000000001 11 1x10-11 0.00000000001 12 1x10-12 0.000000000001 13 1x10-13 0.0000000000001 14 1x10-14 0.00000000000001 8.4 – pH Scale Find pH from H+ pH = -log[H+] Find H+ from pH [H+] = 10-pH pH<7 ACIDIC pH=7 NEUTRAL pH>7 BASIC pH(1) 100x more acidic than pH(3) pH(6) is 10,000x more basic than pH(2)

8.4 – pOH Scale Find pOH from OH- pOH = -log[OH-] Find OH- from pOH pH pOH [OH-] ions in scientific notation [OH-] ions traditionally 14 1x10-14 0.00000000000001 1 13 1x10-13 0.0000000000001 2 12 1x10-12 0.000000000001 3 11 1x10-11 0.00000000001 4 10 1x10-10 0.0000000001 5 9 1x10-9 0.000000001 6 8 1x10-8 0.00000001 7 1x10-7 0.0000001 1x10-6 0.000001 1x10-5 0.00001 1x10-4 0.0001 1x10-3 0.001 1x10-2 0.01 1x10-1 0.1 1x100 1.0 8.4 – pOH Scale Find pOH from OH- pOH = -log[OH-] Find OH- from pOH [OH-] = 10-pOH pOH<7 BASIC pOH=7 NEUTRAL pOH>7 ACIDIC pH + pOH = 14 [H+][OH-] = 1x10-14

8.4 – Properties of Water Water is neutral, duh!! It’s neutral because it has equal amounts of both [H+] = 1x10-7 [OH-] = 1x10-7 pH = 7 pOH = 7 This allows us to use water as a reference point pH + pOH = pKw [H+][OH-] = Kw

8.4 – pH vs pOH You now have the ability to convert between any of the four, pH, pOH, H+, and OH- We now have a way to relate pH and pOH pH + pOH = pKw = 14 [H+][OH-] = Kw = 1x10-14 -log[H+] = pH -log[OH-] = pOH 10-pH = [H+] 10-pOH = [OH-]

8.4 – pH Probe and meter pH meter is used to measure pH The voltage difference is measured between the acid inside the bulb and that outside. This is converted into a pH reading by the meter to very high resistance electric voltmeter via coaxial cable Reference electrode, e.g. Ag/AgCl Platinum wire in buffer solution Porous plug that acts as a salt bridge Thin glass bulb Solution under test