Mr. Bouyer
Day 1 - 2 | Day 3 - 4 | Day 5 | Lab | Skills Test

Use the hydronium ion concentration of a solution to calculate pH. Use the hydroxide ion concentration of a soultion to calculate pOH. When given either pH or pOH, calculate the other. Use indicators to determine the pH of a solution. Predict the characteristics of a solution produced by hydrolysis.

Developing the concept of pH:

Water ionizes according to the equation: 2H₂O_(l) H₃O⁺_(aq) + OH ^-_(aq)
The equalibrium expression for the reaction would be: K_eq = [H₃O⁺] [OH ^-] / [H₂O]²
Experimental evidence indicates that pure water contains 1 X 10 ^-7 mole of both H₃O⁺ and OH ^-
The concentration of water in pure water is calculated as 55.6 moles/dm³
This information allows us to get an expression: K_sp(55.6)² = [H₃O⁺] [OH ^-] = 1 X 10 ^-14
K_sp(55.6)² becomes a new constant, the ion product constant of water, K _w

The expression becomes

K w = [H3O+] [OH -] = 1 X 10 -14

The importance of K _w

• K _w is a constant for all dilute aqueous solutions at room temperature.
• Although the [H₃O⁺] and [OH ^-] may change, the product is always 1 X 10 ^-14
• This provides the basis for the pH scale.

Acid strength increases	7	Base strength increases
The "strength" of an acid or base increases with distance from pH=7

pH of some common substances:

Calculating pH:

• The equation is: pH = - log [H₃O⁺]
• [H₃O⁺] is expressed in powers of 10 from 10 ^-14 to 10 ⁰
• If [H₃O⁺] = 1 X 10 ^-7, the negative log of [H₃O⁺] = 7. The pH equals 7, indicating a neutral solution.
• The calculation of pH always gives a number between 0 and 14.

Sample pH Calculations: Two ways are shown to work many of these problems: The left column shows the problem worked using logarithm tables found on pages 865 and 866 in your textbook. The right column shows the problems worked using a calculator. You are expected to understand logarithm tables, but you are allowed to work all problems in this class using a calculator. Remember that you must show the equation to be used, then the numbers plugged into the equation, then the answer circled. What is the pH of a solution with a [H3O+] of 1.00 X 10 -4 M? Using Log Tables Using Calculator pH = - log [H3O+] pH = - log (1 X 10 -4) pH = - (log 1.00 + log 10 -4) pH = - (0 + (-4)) pH = - (-4) pH = 4 pH = - log [H3O+] pH = - log (1 X 10 -4) Do the following on your calculator: Enter 1 X 10 -4 Press " LOG " Press " +/- " Hint: when the interger of the scientific notation is 1, the pH is the exponent of 10 as a positive number. pH = 4 0.01 moles of HCl is added to water to make 1dm3 of solution. Assuming the HCl is completely ionized, what is the pH of the solution? Using Log Tables Using Calculator [H3O+] = 1 X 10 -2 M pH = - log [H3O+] pH = - log (1 X 10 -2) pH = - (log 1.00 + log 10 -2) pH = - (0 + (-2)) pH = - (-2) pH = 2 pH = - log [H3O+] pH = - log (1 X 10 -2) Do the following on your calculator: Enter 1 X 10 -2 Press " LOG " Press " +/- " Hint: when the interger of the scientific notation is 1, the pH is the exponent of 10 as a positive number. pH = 2 Calculate the [H3O+] of a solution with a pH of 3.70. Using Calculator pH = - log [H3O+] - pH = log [H3O+] . . . . (you may begin with this equation) -3.70 = log [H3O+] antilog -3.70 = [H3O+] Do the following on your calculator: Enter -3.70 antilog is usually "INV", "shift", or "2nd" + "log" on a calculator. Now is the time to find out which of these it is on your calculator. [H3O+] = 2 X 10 -4 M

• pH meter:
  • An electronic device that measures pH directly.
  • pH meters are used in most professional lab settings today.
• Indicators:
  • Weak organic acids and bases whose colors differ from the colors of their conjugate acids or bases.
  • The color is best viewed from above against a white background.

    Acid / Base Indicators
    Indicator	Lower Color	pH Range	Upper Color
    methyl violet	yellow-green	0.0 - 2.5	violet
    methyl orange	red	2.5 - 4.4	yellow
    congo red	blue	3.0 - 5.0	red
    bromocresol green	yellow	4.5 - 5.5	blue
    methyl red	red	4.8 - 6.0	yellow
    bromocresol purple	yellow-green	5.4 - 6.8	violet
    bromothymol blue	yellow	6.0 - 7.6	blue
    phenol red	yellow	6.4 - 8.2	red-violet
    cresol red	yellow	7.1 - 8.8	violet
    phenolphthalein	colorless	8.3 - 10.0	dark pink
    Phenolphthalein is a common indicator to use in neutralization titrations. The solution is considered neutral when the solution holds a very faint pink color for half a minute or more.
    alizarin yellow R	yellow	9.9 - 11.8	dark orange

  • Universal indicator solution has a wide range of color changes:
    
    
  • Hydrion paper:
    • A paper that goes through changes similar to the universal indicator solution.
  • Litmus paper:
    • Red litmus paper turns blue in a base.
    • Blue litmus paper turns red in an acid.
    • There is a litmus liquid with similar color response.

Buffer systems:

• Solutions that can absorb moderate amounts of acid or base without a significant change in pH.
• Buffers provide ions that react with H₃O⁺ or OH ^-, if they are introduced into the solution. As either ion is neutralized, the pH of the system remains nearly constant.
• Buffer solutions are prepared using a weak acid or base with one of its salts. The weak acid or base reacts with one of the added ions, the salt reacts with the other ion. The combination keeps the pH fairly constant, up to a point.
• By choosing the correct weak acid or base, a buffer solution can be prepared that will maintain almost any pH.

Use these procedures to make solutions ranging in pH from 1 to 13. Place a small amount of each solution in well plates sitting on white paper. Add one drop of universal indicator to each well. Record the color of universal indicator at each pH. Take a digital photograph of the well plates that can be used as a reference.

Day 5

Hydrolysis: the reaction of a salt with water to form an acidic or basic solution.

The type of soultion produced when a salt dissolves in water depends on the properties of the acid and base that combine to form the salt. The following table summarizes hydrolysis.

To use this table to predict the resulting solution from hydrolysis, one must be able to recognize strong and weak acids and bases.

Research References:

• pH - Diamond Bar High School ChemTeam
• Solution Preparations - Frontier High School

Chemistry Class

Answers to "Try These"

1. 5
2. 1 X 10 ^-9 M
3. 2.5
4. 12
5. 1.26 X 10 ^-9 M

Practice Problem Answers:

1. pH = 7.02
2. [H₃O⁺] = 3.5 X 10 ^-6 M
3. pH = 12.64
4. pOH = 9.53
5. pH = 10

Practice Problem Answers:

1. acidic
2. acidic
3. neutral
4. acidic
5. acidic
6. basic
7. basic