pH at a glance

 

DEFINITION OF pH

pH is defined as the negative logarithm to the base 10 of the hydrogen ion [H⁺] concentration.

It is also defined as the degree of acidity or alkalinity of a solution

i.e. pH = -log₁₀ [H⁺].

Thus: If [H⁺] = 0.00001 or 10^-5.

log [H⁺] = log10^-5 = -5

pH= -log [H⁺] = - (-5) = 5.

If [H⁺] =10^-x

Therefore, pH= -log10^-x = - (-x) = x

If [H⁺] = 10^-2, pH = 2

DEFINITION OF pOH

pOH is defined as the negative logarithms of the hydroxide ion [OH^-] concentration to the base of 10.

i.e. pOH= -log [OH^-].

A solution with pH 7 is neutral.

A solution with pH less than 7 i.e. pH 6,5,4, 3, 2, 1 or 0 indicates increasing acidity as the numbers decreases.

A solution with pH greater than 7, i.e. pH 8,9,10, 11, 12, 13, or 14 indicate increasing alkalinity as the numbers increase.

 1< 2 <3< 4< 5< 6        7          8< 9<10< 11<12<13<14

Increasing acidity    Neutral     Increasing alkalinity

A solution with pH 1 is very acidic [with high concentration of H⁺]. A solution with pH 13 is very alkaline [with low concentration of H+, but high concentration of OH-].

Note that: If pH is 1, it has concentration of H+ 10 times greater than pH 2 and 100 times greater than pH 3 e.t.c.

pH 1 > pH 2 > pH 3.

Concentration of H⁺    10^-1      10^-2      10^-3.

                                      0.1       0.01     0.001.

Relationship between pH and pOH.

                 H₂O →   H⁺    +   OH^-

From conductivity measurement 

[H⁺] =10^-7moldm^-3, [OH^-]=10^-7moldm^-3.

[H⁺] [OH^-] = Kw=10^-7 x 10^-7=10^-14mol²dm^-6.

Taking logarithm of both sides

log ([H⁺] [OH^-]) = logKw

log [H⁺] + log[OH^-] =logKw

Subtracting both sides

-(log[H⁺]  +  [OH^-]) = -logKw

-log [H+] – log[OH^-] = -logKw

-log [H⁺] + (-log [OH^-]) = -logKw

pH + pOH = PKw

pKw = -log10^-14 = -(-14) = 14

Therefore, pH + pOH = 14.

Worked examples

1. Find the hydrogen and hydroxide ion concentrations in

(a) 0.01moldm^-3 tetraoxosulphate (vi) acid solution.

(b) 0.001moldm^-3 potassium hydroxide solution.

Solution

(a).  H₂SO_4(aq)→ 2H⁺_(aq)+SO₄^2-_(aq)

From the equation, 

1 moldm^-3 H₂SO₄ ionizes to give 2moldm^-3 H⁺

Therefore, 0.01moldm^-3 H₂SO₄ would ionize to give (2x0.01) moldm^-3 H⁺

[H⁺] = 2x10^-2moldm^-3

[H⁺] [OH^-] = 10^-14

(2x10^-2) [OH^-] = 10^-14

[OH^-] =    10^-14

                2 x 10^-2

[OH-] = 0.5x (10^{-14- -2})

[OH-] =0.5 x10^-14+2

[OH-] =0.5x10^-12moldm^-3.

(b).     KOH_(aq)→ K⁺_(aq)+  OH^-_(aq)

From the equation,

1moldm^-3 of KOH ionizes to give 1moldm^-3 of OH-

10^-3moldm^-3 of KOH would ionize to give 10^-3moldm^-3 of OH

[OH^-]=10^-3moldm^-3.

[H⁺] [OH^-]=10^-14

[H⁺] (10^-3) = 10^-14.

[H⁺] = 10^-14

            10^-3

[H+] = 10^-14+3

[H+] = 10^-11moldm^-3

2.  A glass cup of orange juice is found to have a POH of 11.40. Calculate the concentration of the hydrogen ions in the juice.

Solution

pH + pOH = 14.

pH = 14 – 11.4.

pH = 2.6.

pH = -log [H⁺]

2.6 =-log [H⁺].

[H+] = Antilog (-2.6)

[H+] = 0.0025moldm^-3

[H+] = 2.5x10^-3moldm^-3.

Measuring pH of a solution.

We use the pH meter or a universal indicator to measure the pH of a solution. 

A universal indicator shows different colours at specific pH or hydrogen ion concentrations.

                          

OBJECTIVE QUESTIONS

1. What does pH measure?
A. Temperature of a solution
B. Acidity or alkalinity of a solution
C. Mass of a solution
D. Density of a solution 

2. A solution with pH 7 is said to be:
A. Acidic
B. Neutral
C. Basic
D. Salty

3. Which of the following has a pH less than 7?
A. Base
B. Alkali
C. Acid
D. Salt

4. Which substance is most likely to have a pH of 2?
A. Soap solution
B. Pure water
C. Lemon juice
D. Sodium hydroxide

5. The pH of pure water at room temperature is:
A. 0
B. 5
C. 7
D. 14

6. A solution with pH 10 is:
A. Strongly acidic
B. Weakly acidic
C. Neutral
D. Alkaline

7. Which of the following is the strongest acid?
A. pH 6
B. pH 4
C. pH 2
D. pH 1

8. Which indicator is commonly used to test pH?
A. Litmus paper
B. Sand paper
C. Filter paper
D. Graph paper

9. As the pH value decreases, the acidity:
A. Decreases
B. Increases
C. Remains constant
D. Becomes neutral

10. Which of the following is alkaline?
A. Vinegar
B. Rain water
C. Soap solution
D. Orange juice

11. The pH scale usually ranges from:
A. 1–7
B. 0–14
C. 0–10
D. 7–14

12. A substance with pH 13 is likely to be:
A. A strong acid
B. A weak acid
C. A strong base
D. Neutral

13. Which of the following household substances is acidic?
A. Toothpaste
B. Baking soda solution
C. Vinegar
D. Detergent

14. What happens to the pH of a solution when acid is added?
A. It increases
B. It decreases
C. It becomes 7
D. It remains unchanged

15. Which of these has the highest pH?
A. Battery acid
B. Distilled water
C. Soap solution
D. Orange juice