Acid/Base Chemistry
The pH scale and what does it mean?
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pH
• What is it?
• What does it mean?
• How do you measure it?
• Can you determine other things from pH?
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pH an Overview
pH is the measure of the concentration of hydrogen ions is a solution. pH is measured on a log scale.
pH = - log[H+] pH is used to indicate the degree of acidity or
alkalinity (basic) of a solution. A solution is of a water solvent (polar)
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When there is a high [H+], the solution is acidic. On the other hand when the [H+] concentration is
low, the solution is basic.
So if there is a high [H+], there must be a low [OH-] If there is a low [H+], there must be a high [OH-]
So what does H+ and OH- create?
H2O
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Water
Pure water should have a neutral pH, due to have equal amounts of [H+] & [OH-].
Acid and bases are created by adding polar molecules to water to increase [H+], or increase [OH-]
The pH scale is used to measure the acidity or alkalinity of a solution.
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pH ScaleThe pH scale ranges from 0 to 14. 0 being the most acidic and 14 being the most basic.
So what does the pH tell us about [H+] of a solution?
If something has a pH of:pH 1 = [H+] = 0.1 MpH 2 = [H+] = 0.01 MpH 3 = [H+] = 0.001MpH 4 = [H+] = 0.0001MpH 5 = [H+] = 0.00001MpH 6 = [H+] = 1X 10-6MpH 7 = [H+] = 1X 10-7MpH 8 = [H+] = 1X 10-8MpH 9 = [H+] = 1X 10-9MpH 10=[H+] = 1X 10-10MpH 11=[H+] = 1X 10-11MpH 12=[H+] = 1X 10-12MpH 13=[H+] = 1X 10-13MpH 14=[H+] = 1X 10-14M
Representative pH Values
Substance pH
battery acid 0.5
gastric acid 1.5-2.0
lemon juice 2.4
cola 2.5
vinegar 2.9
orange juice 3.5
acid rain 4.5-5.0
tea 5.5
milk 6.5
pure water 7.0
saliva 7.0-7.4
blood 7.3-7.5
sea water 8.0
soap 9.0-10.0
ammonia 11.5
household bleach 12.5
lye (sodium hydroxide) 13.5
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How to Figure it out?
If you were given HCl with a concentration of 0.003M, what would be the pH?
pH = -log [H+]
pH = -log[0.003M]
pH = 2.52
pH will not have a unit!!!
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How does pH and pOH relate?
pH and pOH relate in the following way
pH + pOH = 14
pOH is the measure of the [OH-], in a solution.
What would be the pOH of an solution with a pH of 5.75?
pOH = 14 - 5.75
pOH = 8.258Created by Seth Floyd - What's on Your
Shelf?
If given pH
To determine the concentration from pH, one must do the following:
[H+] = 10 (-pH)
So what is the [H+] of a solution with a pH of 12.05?
[H+] = 10 (-12.05)
[H+] = 8.91 X 10 -13 M
antilog
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Sample Problem 1
What is the pH and pOH of an solution with [H+] of 3.33 X 10-9 M?
1st solve for pH pH = -log [H+]
pH = - log[3.33 X 10-9 M]
pH = 8.48
2nd solve for pOH pOH = 14 – pH
pOH = 14 - 8.48
pOH = 5.52 (acidic)10Created by Seth Floyd - What's on Your
Shelf?
Sample Problem 2
What is the [H+] of a solution with a pOH of 4.14?
1st determine pH pH = 14 – pOH
pH = 14 – 4.14
pH = 9.86 (basic)
2nd determine [H+] [H+] = 10 (-pH)
[H+] = 10 (-9.86)
[H+] = 1.38 X 10 -10 M11Created by Seth Floyd - What's on Your
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Sample Problem 3
What is the pH of a solution with a [OH-] = 2.35 X 10-9?
1st determine pOH pOH = -log [OH-]
pOH = -log[2.35X10-9 M]
pOH = 8.63
2nd determine pH pH = 14 – pOH
pH = 14- 8.63
pH = 5.37 (acidic)12Created by Seth Floyd - What's on Your
Shelf?
Sample Problem 4
What is the pH of a solution that was made from 4.57g of HCl dissolved in 750 mL of water?
1st determine moles of HCl
4.57g HCl
36.46 g HCl
1 mol HCl = 0.125 mol HCl
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Sample 4 ….
2nd determine concentration of [H+], since this is an acid the [H+] would be the same as the concentration of the whole molecule of HCl. We will get [H+] & [Cl-], both with the total of 0.125 moles each.
So concentration will be moles / volume (L)
[H+] = 0.125 moles/ 0.750 L
[H+] = 0.333M
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Problem 4…
Next determine pH
pH = -log [H+]
pH = -log [0.333M]
pH = 0.48 (very acidic)
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Your Turn..
1. What is the [H+] of an acid with a pH of 5.56?
2. Determine the pH of a 0.0045 M HCl?
3. Determine pOH of a solution with a pH of 8.65?
4. What is the pH of a base with an [OH-] of 3.44X10-
5 M?
Ans: 1. 1.35
2. 2.75X 10-6M
3. 5.35
4. 9.5416Created by Seth Floyd - What's on Your
Shelf?