Question

The pH plays an important role in our daily life. Strength of acids and bases depends on the number of H+ ions and OH− ions produced by these respectively. pH scale helps in predicting the strength of acids and bases by measuring hydrogen ion concentration in a solution.

Answer the following:

(a) What type of indicator is used to predict how strong an acid or base is?    [1]

(b) Three solutions A, B and C showed pH as 3, 5 and 7. Which solution has the highest concentration of H⁺ ions?    [1]

(c) What are strong acids and weak acids?

OR

(c) (i) Under what condition is rainwater called acid rain?    [2]

(ii) In the following acids, identify weak acids:

Nitric acid, hydrochloric acid, acetic acid, sulphuric acid, formic acid.    [2]

Answer 1

(a) Universal indicator is used to predict how strong an acid or base is.

(b) A solution A (pH = 3) has the highest concentration of H⁺ ions (lower pH has higher acidity).

(c) Strong acids: Acids that completely ionise in aqueous solution (e.g., HCl, H₂SO₄).
Weak acids: Acids that partially ionise in aqueous solution (e.g., CH₃COOH).

(c) (i) Rainwater is called acid rain when its pH is less than 5.6, due to dissolved acidic gases like SO₂ and NO₂.

(ii) Weak acids are acetic acid and formic acid.