pH Scale and Indicators - UNSOLVED PRACTICE SET

Q7. What is the pH scale? What is the pH of: (i) acidic solutions, (ii) basic solutions, and (iii) neutral solutions?

Q8. What is an indicator? Name two natural indicators and two synthetic indicators. What colour change do they show in acidic and basic media?

Q9. Your mother uses turmeric paste as a home remedy for burns. She notices that when she applies soap (basic) to turmeric-stained cloth, the stain turns red. Explain this observation.

Q10. The pH of gastric juice in the stomach is about 1-3, while the pH of blood is about 7.4. What does this tell you about the nature of these fluids? Why is it important for blood pH to remain constant?

Q11. How does the pH scale help in comparing the strength of acids and bases? Compare the acidity of a solution with pH 2 and a solution with pH 5.

Q12. Litmus is a natural indicator extracted from lichens. Explain why litmus solution is purple (neutral) and how it changes colour in acidic and basic solutions.

Q13. Explain the pH scale with a labelled diagram. How does the concentration of hydrogen ions [H⁺] and hydroxide ions [OH⁻] change as we move from pH 0 to pH 14? Describe the colour changes shown by litmus, phenolphthalein, and methyl orange in acidic, neutral, and basic solutions. Why is the pH scale important in everyday life?

Q14. A student is given five solutions with pH values 2, 5, 7, 9, and 12. She has universal indicator paper, phenolphthalein, and methyl orange. Help her by:

(a) Predicting the colour of universal indicator paper for each solution.

(b) Predicting the colour of phenolphthalein in each solution.

(c) Predicting the colour of methyl orange in each solution.

(d) Calculating the [H⁺] concentration for the solution with pH 2 and pH 12.

(e) Explaining how many times more acidic the pH 2 solution is compared to the pH 5 solution.

Q15. "The pH scale is not just a chemistry concept — it is a tool that affects our health, our environment, and our food." Discuss this statement by explaining: (i) the importance of pH in human body fluids, (ii) the pH of common foods and beverages and their effect on health, (iii) the environmental impact of acid rain and its pH, and (iv) how farmers use pH to determine soil quality for crop growth.

Q16. A chemistry teacher prepares solutions with different pH values for a class demonstration:

Table

Solution pH

A 2

B 5

C 7

D 10

E 13

(a) Calculate the [H⁺] concentration for each solution.

(b) Calculate the [OH⁻] concentration for solutions D and E. (Kw = 10⁻¹⁴ at 25°C)

(c) How many times more acidic is solution A compared to solution B?

(d) If solution C is pure water, what happens to its pH when CO₂ from air dissolves in it? Explain.

(e) A patient has stomach acidity with pH 1.5. Calculate the [H⁺] concentration. How many times more acidic is this than solution A?

Q17. In a school science fair, students test the pH of various household substances:

Table

Substance pH

Lemon juice 2.2

Tomato juice 4.5

Milk 6.6

Pure water 7.0

Egg white 8.0

Baking soda solution 8.5

Soap solution 9.0

Drain cleaner 13.0

(a) Classify each substance as strongly acidic, weakly acidic, neutral, weakly basic, or strongly basic.

(b) Calculate the [H⁺] concentration in lemon juice and in drain cleaner.

(c) How many times more basic is drain cleaner (pH 13) compared to baking soda solution (pH 8.5)?

(d) A student mixes equal volumes of lemon juice (pH 2.2) and baking soda solution (pH 8.5). Predict whether the resulting mixture will be acidic, basic, or neutral. Explain your reasoning.

(e) Why is it dangerous to use drain cleaner without gloves, even though both drain cleaner and soap solution are basic?

Q18. A water treatment plant in India monitors the pH of drinking water before distribution.

(a) The World Health Organization recommends drinking water pH between 6.5 and 8.5. If a water sample has pH 6.2, is it safe for drinking? What should be done?

(b) If the plant treats 1 million litres of water daily and the pH needs to be raised from 6.2 to 7.0, calculate how many moles of OH⁻ ions need to be added per litre. (Assume the change is due only to [H⁺] neutralization)

(c) If Ca(OH)₂ is used for treatment, calculate the mass needed daily. (Molar mass of Ca(OH)₂ = 74 g/mol)

(d) In some parts of India, groundwater has pH below 6 due to acid rain and industrial pollution. What health problems can this cause?

(e) If a village water supply has pH 5.5, calculate how many times more acidic it is than the recommended lower limit of pH 6.5.