# pOH Concepts 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 BasepOH < 7 neutralpOH=7 AcidpOH > 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 neutralpOH=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

pOHExamples (the pOH, at 25°C, is only an approximation in most of these examples)
01 mol L-1 NaOH(aq)
1caustic oven cleaners, 0.1 mol L-1 NaOH(aq)
2cleaning products
3washing soda, some detergents
4toothpaste, some detergents
5some soaps, baking soda (sodium bicarbonate, bicarbonate of soda, or sodium hydrogen carbonate, NaHCO3)
6sea water
7pure liquid water, aqueous sodium chloride solution (NaCl(aq))
8tap water, saliva, cow's milk, urine
9coffee, tomato juice, orange juice
10wine, beer, aspirin dissolved in a glass of water
11lemon juice (contains citric acid), vinegar (contains acetic acid, ethanoic, acid, CH3COOH), coca cola (contains carbonic acid, H2CO3)
12Gastric juice (0.01 mol L-1 HCl(aq)), ant venom (contains formic acid, methanoic acid, HCOOH)
130.1 mol L-1 HCl(aq)
141 mol L-1 HCl(aq)

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## 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 25oC:

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

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1. "Strong" and "weak" are used to describe the strength of a base.
The strength of a base is related to its ability to accept protons NOT to the concentration of hydroxide ions in the solution (pOH).