pOH Concepts Chemistry Tutorial

Key Concepts

• pOH is a measure of the hyroxide ion concentration in a solution.
• pOH can be used to describe the basicity (or akalinity) of a dilute aqueous solution at 25°C :

  pOH=0

  pOH=1

  pOH=2

  pOH=3

  pOH=4

  pOH=5

  pOH=6

  pOH=7

  pOH=8

  pOH=9

  pOH=10

  pOH=11

  pOH=12

  pOH=13

  pOH=14

  Most Basic

  ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ←

  Least Basic

• The lower the value of the pOH of a solution, the more basic (akaline) the solution is.
• pOH can also be used to describe how acidic a solution is:

  pOH=0

  pOH=1

  pOH=2

  pOH=3

  pOH=4

  pOH=5

  pOH=6

  pOH=7

  pOH=8

  pOH=9

  pOH=10

  pOH=11

  pOH=12

  pOH=13

  pOH=14

  Least Acidic

  → → → → → → → → → → → → → → → → → → → → → → → → → → → →

  Most Acidic

  The greater the value of the pOH of the solution, the more acidic the solution is.

• For dilute aqueous solutions at 25°C:
  A base (alkali) has a pOH less than 7.
  An acid has a pOH greater than 7.
  If the solution has a pOH of 7 it is said to be neutral, that is, the solution is neither an acid nor a base.

  pOH=0

  pOH=1

  pOH=2

  pOH=3

  pOH=4

  pOH=5

  pOH=6

  pOH=7

  pOH=8

  pOH=9

  pOH=10

  pOH=11

  pOH=12

  pOH=13

  pOH=14

  Base pOH < 7

  neutral pOH=7

  Acid pOH > 7

• Sometimes we might want to describe the relative acidity, or basicity (alkalinity), of a solution.
  For this we can use the following terms:

  pOH=0

  pOH=1

  pOH=2

  pOH=3

  pOH=4

  pOH=5

  pOH=6

  pOH=7

  pOH=8

  pOH=9

  pOH=10

  pOH=11

  pOH=12

  pOH=13

  pOH=14

  Very Basic

  Basic

  Slightly Basic

  neutral pOH=7

  Slightly Acidic

  Acidic

  Very Acidic

  Avoid using the terms "strong" or "weak" to describe the relative pOH of acids or bases¹.

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pOH of Some Common Solutions

Worked Examples

Question 1. Which of the following substances is a base at 25°C?

• tap water
• orange juice
• vinegar found in the kitchen
• oven cleaner

1. Estimate the pOH of each solution using the table above:

   solution	pOH
   tap water	8
   orange juice	9
   vinegar	11
   oven cleaner	1

2. Use the pOH to determine whether each solution is an acid or a base at 25°C:

   1. base : pOH < 7
   2. neutral : pOH = 7
   3. acid : pOH > 7

   solution	pOH
   tap water	8	pOH > 7	acid
   orange juice	9	pOH > 7	acid
   vinegar	11	pOH > 7	acid
   oven cleaner	1	pOH < 7	base

   Only oven cleaner has a pOH less than 7 so it is a base.

Question 2. Place the following substances, at 25°C, in order from least basic to most basic:

• toothpaste
• lemon juice
• cow's milk
• ant venom

1. Estimate the pOH of each substance at 25°C:

   solution	pOH
   toothpaste	4
   lemon juice	11
   cow's milk	8
   ant venom	12

2. Place the substances in order of pOH from high pOH to low pOH

   solution	pOH
   ant venom	12	highest pOH
   lemon juice	11	↓
   cow's milk	8	↓
   toothpaste	4	lowest pOH

3. Use the pOH values to determine the relative basicity (alkalinity) of the solutions:
   low pOH is more basic (alkaline) than high pOH

   solution	pOH
   ant venom	12	highest pOH	least basic
   lemon juice	11	↓	↓
   cow's milk	8	↓	↓
   toothpaste	4	lowest pOH	most basic

   The substances in order from least basic to most basic are:

   ant venom, lemon juice, cow's milk, toothpaste

Question 3. The pOH of an unknown solution was measured at 25°C and found to be 7.
The solution is known to be either white wine, white vinegar or table salt (sodium chloride) dissolved in water.
Identify the unknown solution.

1. Estimate the pOH of each the named solutions at 25^oC:

   solution	pOH
   white wine	10
   white viengar	11
   dissolved table salt (NaCl)	7

2. Identify the solution with a pOH = 7 : dissolved table salt

   The unknown solution is most likely to be table salt dissolved in water because both wine and vinegar are acids with pOH > 7