Chemistry: pH

Sunshine State Standards:SC.A. 1.4.2, SC.A.2.4.5, SC.C.2.4.2, and SC.C.2.4.5

pH is a measure of the [H₃O ⁺] in solution.

Developing the concept of pH:

Water ionizes according to the equation: 2H₂O_(l) H₃O⁺_(aq) + OH ^-_(aq)
The equilibrium expression for the reaction would be: K_eq = [H₃O⁺] [OH ^-] / [H₂O]²
Experimental evidence indicates that pure water contains 1 X 10 ^-7 moles of both H₃O⁺ and OH ^-
The concentration of water in pure water is calculated as 55.6 moles/L
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 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.

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: Using log tables (the hard way). Using a scientific calculator (the easy way). You are expected to understand the concept of logarithms, but you are allowed to work all problems in this class using a calculator. Different types of calculators require different key punch entries. The following key punch instructions are for "typcial" calculators. Graphing calculators require different keying. Remember that the "ee" or "exp" buttons should be used anytime the "x 10" appears in a calculation. 1. 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 2. 0.01 moles of HCl is added to water to make 1 L 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 3. 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

 

 

pOH is a measure of the [OH ^-] in solution.

Calculating pOH:

• The equation is: pOH = - log [OH ^-]
• [OH ^-] is expressed in powers of 10 from 10 ^-14 to 10 ⁰
• [H₃O⁺] [OH ^-] = 1 X 10 ^-14
• pH + pOH = 14

 

 

  Determining pH experimentally:

• 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.

• 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.

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

The type of solution 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.

 

 

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