IB Chemistry on Acid Base Dissociation Constant, pH scale and Kw of water
Water as and Acid and a Base pH Scale
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Transcript of Water as and Acid and a Base pH Scale
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Pg 569 - 581
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Amphoteric substance: can act as an acid or as a base◦ Water is the most common amphoteric substance
Self-ionization of water:
H2O(l) + H2O(l) H3O+(aq) + OH-(aq)
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At 25oC actual concentrations are:◦ [H3O+] = [OH-] = 1.0 x 10-7 M
The product of the two concentrations is:◦ [H3O+]*[OH-] = 1.0 x 10-14 @ 25oC
We call this Kw
To simplify the equation, we write H3O+ as H+
Kw = [H+]*[OH-] = 1.0 x 10-14 @ 25oC
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Kw = ion-proton constant◦ In any aqueous solution at 25oC, no matter what it
contains, the product of [H+] and [OH-] must ALWAYS equal 1.0 x 10-14
What will happen if the concentration of H+ increases? If it decreases?◦ If [H+] increases, [OH-] decreases◦ If [H+] decreases, [OH-] increases
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1. [H+] = [OH-] neutral2. [H+] > [OH-] acidic3. [H+] < [OH-] basicRemember: [H+]*[OH-] = 1 x 10-14
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Calculate [H+] or [OH-] as required for each of the following solutions at 25 °C, and state whether the solution is neutral, acidic, or basic.
a) 1.0 x 10-5 M OH-
b) 1.0 x 10-7 M OH-
c) 10.0 M H+
H+ = 1.0 x 10-9 M, basic
H+ = 1.0 x 10-7 M, neutral
OH- = 1.0 x 10-15 M, acidic
Calculate [H+] in a solution in which [OH-] = 2.0 x 10-2 M. Is this solution acidic, neutral or basic?
5.0 x 10-13 M, basic
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pH measures the “power” of the hydrogen ion◦ “Power” refers to exponent of 10◦ Based on the base 10 log scale.
pH = -log[H+] If [H+]= 1.00 x 10-7, pH = ? If [H+]= 1.00 x 10-2, pH = ?
[H+] = inverse log (-pH) If pH = 9, [H+]=?
If a solution has a pH of 2, how much more acidic is it than a solution with a pH of 4? 100 times more acidic!!
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Try the following:1. Find pH if [H+] = 3.5 x 10-3
Find pH if [OH-] = 1.4 x 10-3
2. If the pH = 9.4 what is [H+] ?If the pH = 4.9 what is [OH-] ?
Answers: 1) 2.45 2) 11.15 3) 4.0 x 10-10 4) 7.9 x 10-10
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pOH = -log[OH-] If pH = 5, What would pOH be?
pH + pOH = 14.00
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1. The pOH of a solution is 5.67. What is the [OH-] of the solution? What is the [H+]?
2. The [OH-] of drain cleaner is 3.2 x 10-11M. What is the pOH? The pH? Is it acidic, basic or neutral?
1) 2.1 x 10-6M and 4.8 x 10-9M
2) 10.50 and 3.50 = acidic!!