JSS 2: pH

                              pH

pH stands for the potential of hydrogen. It is a measure of the degree of acidity or alkalinity (basicity) of a solution. It is measured by an instrument called pH meter.

                         pH SCALE 

The pH scale ranges from 0 to 14; the lower and upper limits respectively.

Note:

1. Substances with pH < 7 is  Acidic e.g.battery acid, vinegar, gastric juice etc

2. Substances with pH = 7 is Neutral e.g. Pure water, table salt etc

3. Substances with pH > 7 is Alkaline (Basic) e.g. Bile, sea water, soap solution, NaOH etc

               Importance of pH

1. pH helps to determine whether a substance is acidic or alkaline.

2. It is used in agriculture to test soil before planting crops.

3. Used in medicine to monitor body fluids.

4. Helps in water treatment and swimming pool maintenance.

5. Used in food and beverage production.

6. Important in chemical industries and laboratories.

                      Indicators

Indicators are substances that change colour in acidic or basic solutions.

Examples of common Indicators:

1. Universal indicator 

2. Litmus paper 

3. Methyl orange 

4. Phenolphthalein 

          Importance of Indicators

1. To detect acids and bases.

2. To determine the pH of substances.

             

                Calculations on pH and pOH

pH is the negative logarithm to base 10 of the hydrogen ion [H⁺] or hydroxonium ion [H₃O⁺] concentration.

pOH is the negative logarithm to base 10 of the hydroxyl ion [OH⁻] concentration.

Formulae:

1. pH = –log₁₀[H⁺]  or  pH = –log₁₀[H₃O⁺]

2. pOH = –log₁₀[OH⁻]

3. pH + pOH = 14

4. [H⁺] x [OH⁻] = 10⁻¹⁴

1. Calculate the pH of a solution whose hydrogen ion concentration is 1×10⁻³ mol/dm³.

Solution:

pH = –log₁₀[H⁺]

pH = –log₁₀[1×10⁻³]

pH = –log₁₀[10⁻³]

pH = –(-3)log₁₀10 (Note: log₁₀10= 1)

pH = –(-3)x1

pH = 3 × 1

pH = 3

2. Calculate the pOH of a solution whose hydroxyl ion concentration is 1×10⁻⁹ mol/dm³.

Solution:

pOH = –log₁₀[OH⁻]

pOH = –log₁₀[1×10⁻⁹]

pOH = –log₁₀[10⁻⁹]

pOH = –(-9)log₁₀10 

pOH = –(-9)x1

pOH = 9 × 1

pOH = 9

3. A solution has a pH of 4. Calculate its pOH.

Solution:

pH + pOH = 14

  5  + pOH = 14

         pOH = 14 - 4

         pOH = 10

4. The hydrogen ion concentration of a solution is 1 × 10⁻⁴ mol/L. Calculate its hydroxide ion (OH⁻) concentration.

Solution:

 [H⁺] x [OH⁻] = 10⁻¹⁴

 (1 × 10⁻⁴ ) ×  [OH⁻] = 10⁻¹⁴

 Divide both sides by (1 × 10⁻⁴ )

 [OH⁻] = 10⁻¹⁴/(1 × 10⁻⁴ )

 [OH⁻] = 10⁻¹⁴/ 10⁻⁴

 [OH⁻] = 10⁻¹⁰ mol/L

5. An orange juice has a pH of 4. Calculate its hydrogen ion concentration [H⁺].

Solution:

pH = –log₁₀[H⁺]

   4 = –log₁₀[H⁺]

  -4 = log₁₀[H⁺]

Antilog(-4) = [H⁺]

      [H⁺] = Antilog(-4)

      [H⁺] = 10⁻⁴

6. A pineapple juice has a pH of 3. State whether it is a strong acid, weak acid, neutral substance, or alkali.

Answer: _________________