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

Shelf?

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?