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pOH Concepts Tutorial

Key Concepts

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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?

  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:

  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).