# Todays lesson is brought to you by 7, 14, and the letters Kw, pH.

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Todays lesson is brought to you by 7, 14, and the letters Kw, pH

Kw: the ion-product constant of water Q: will pure distilled water conduct electricity A: As the demo shows, it will (slightly). If water conducts electricity, ions must exist Water exists as an equilibrium, which is referred to as the self-ionization of water: H 2 O + H 2 O H 3 O + (aq) + OH – (aq) Simplified reaction: H 2 O H + (aq) + OH – (aq) Kc = [H 3 O + ] [OH – ] [H2O]2[H2O]2 Kw =or [H 3 O + ] [OH – ] Note: H + is just shorthand for H 3 O + Kc = [H + ] [OH – ] [H2O][H2O] Kw =or [H + ] [OH – ]

Kw What is the value of Kw? It has been measured at 1.0 x 10 –14 (25 °C) Notice (pg. 600) that by definition: when the solution is [H + ] > [OH – ] acidic [H + ] < [OH – ] basic [H + ] = [OH – ] neutral Pure water is neutral since [H + ] and [OH – ] must be identical (both come from one H 2 O) As temperature increases Kw increases Q: Rewrite the equilibrium of water with heat as a product or reactant (based on above point) Q: Do PE 1 (refer to example 15.1)

Answers As temperature increases Kw increases. It must be that: H 2 O + heat H + + OH – (increase temp = shift right = Kw) PE 1 Kw = [H + ] [OH – ] 1.0 x 10 –14 = [H + ] [7.8 x 10 –6 ] [H + ] =1.28 x 10 –9 Thus, the solution is basic: [H + ] < [OH – ]

pH Notice (PE1) that a change in [H + ] is matched by a change in [OH – ], since Kw is constant (see middle columns of table 15.2 – pg. 603) [H + ] is commonly referred to because it is critical to chemical and biochemical reactions A quick method of denoting [H + ] is via pH By definition pH = – log [H + ] The pH scale, similar to the Richter scale, describes a wide range of values An earthquake of 6 is10x as violent as a 5 Thus, the pH scale condenses possible values of [H + ] to a 14 point scale Also, it is easier to say pH = 7 vs. [H + ] = 1 x 10 –7

Calculations with pH pH = – log [H + ], what is pH if [H + ] = 6.3 x 10 –5 ? Enter 6.3 x 10 –5 (6.3, exp, 5, +/-) Then hit log, followed by +/- Ans: 4.2 3.98 x 10 –8 M What is [H + ] if pH = 7.4? To solve this we must rearrange our equation [H + ] = 10 –pH mol/L Enter 10, x y, 7.4, +/-, = Finally, notice on pg. 603 that pH + pOH = 14 This is related to Kw = 1.0 x 10 –14 You do not need to know how equations are derived; you need to know how to use them

Equations and practice Read footnote on 601 about significant digits Do PE 2 – 4 on pg. 605 Use examples 15.2 – 15.4 as reference pH = – log [H + ] You will need to memorize the following: pOH = – log [OH – ] [H + ] = 10 –pH [OH – ] = 10 –pOH pH + pOH = 14

PE 2 answers a)pH = – log [H + ] = – log [0.020] = 1.6989 = 1.70 pH + pOH = 14 pOH = 14 – pH pOH = 14 – 1.70 =12.3 b)pOH = – log [OH – ] = – log [0.0050] = 2.3 pH = 14 – pOH = 14 – 2.3 =11.7 c)pH = – log [H+] = – log [7.2x10 -8 ] = 7.14 pOH = 14 – pH pOH = 14 – 7.14 = 6.86 (slightly basic) d) Ba(OH) 2 Ba 2+ + 2OH – pOH = – log [OH – ] = – log [0.00070] = 3.15 pH = 14 - pOH = 14 – 3.15 = 10.85

PE 3 answers a)[H + ] = 10 –pH = 10 –2.90 = 1.29 x 10 –3 mol/L pOH = 14 – pH = 14.00 – 2.90 = 11.1 [OH – ] = 10 –pOH = 10 –11.1 = 7.94 x 10 –12 mol/L b)[H + ] = 10 –pH = 10 –3.85 = 1.41 x 10 –4 mol/L pOH = 14 – pH = 14.00 – 3.85 = 10.15 [OH – ] = 10 –pOH = 10 –10.15 = 7.08 x 10 –12 mol/L c)[H + ] = 10 –pH = 10 –10.81 = 1.55 x 10 –11 mol/L pOH = 14 – pH = 14.00 – 10.81 = 3.19 [OH – ] = 10 –pOH = 10 –3.19 = 6.46 x 10 –4 mol/L

PE 3, 4 answers d)[H + ] = 10 –pH = 10 –4.11 = 7.76 x 10 –5 mol/L pOH = 14 – pH = 14.00 – 4.11 = 9.89 [OH – ] = 10 –pOH = 10 –9.89 = 1.29 x 10 –10 mol/L e)[H + ] = 10 –pH = 10 –11.61 = 2.45 x 10 –12 mol/L pOH = 14 – pH = 14.00 – 11.61 = 2.39 [OH – ] = 10 –pOH = 10 –2.39 = 4.07 x 10 –3 mol/L PE 4 a) acidic, b) acidic, c) basic, d) acidic, e) basic

Measuring pH pH can be measured in several ways Usually it is measured with a coloured acid- base indicator (see pg. 606) or a pH meter Coloured indicators are a crude measure of pH, but are useful in certain applications pH meters are more accurate, but they must be calibrated prior to use Calibration means setting to a standard A pH meter is calibrated with a solution of known pH often called a buffer Buffer indicates that the pH is stable

Using pH meters 1.Always rinse pH meter in distilled water prior to placing it in a solution (buffer or otherwise) 2.Place the pH meter in a buffer with about the same pH as that of your solution (4, 7, or 10) 3.Turn on the pH meter only when in solution Start with with buffer 7. Hit cal once. Wait upto a minute until it automatically sets Rinse, dry, and place in second buffer (4/10) Hit cal once. Wait until it automatically sets There is no need to use read. 4.Measure the pH of your solution For more lessons, visit www.chalkbored.com www.chalkbored.com

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