Arrange the following resultant mixtures in increasing order of their pH values Choose the correct answer from the options given below :
- B < C < A
- C < A < B
- C < B < A
- A < C < B
Answer
(3) C < B < A
Solution
Compare millimoles (mmol = M × mL) of H+ and OH- in each mixture.
A. OH- = 2 × 0.2 × 10 = 4 mmol; H+ = 0.1 × 25 = 2.5 mmol
⇒ excess OH- = 1.5 mmol in 35 mL ⇒ [OH-] = 0.043 M ⇒ pH ≈ 12.6 (basic)
B. H+ = 2 × 0.01 × 10 = 0.2 mmol; OH- = 2 × 0.01 × 10 = 0.2 mmol
⇒ complete neutralisation (strong acid + strong base) ⇒ pH = 7
C. H+ = 2 × 0.1 × 10 = 2 mmol; OH- = 0.1 × 10 = 1 mmol
⇒ excess H+ = 1 mmol in 20 mL ⇒ [H+] = 0.05 M ⇒ pH ≈ 1.3 (acidic)
∴ Increasing pH: C < B < A
Answer: (3)