JEE Main 2025 · 7 April · morning · Single correct · +4 / −1
An aqueous solution of HCl with pH 1.0 is diluted by adding equal volume of water (ignoring dissociation of water). The pH of HCl solution would
( Given log 2=0.30)
- (a) increase to 1.3
- (b) reduce to 0.5
- (c) increase to 2
- (d) remain same
Answer and solution
Answer (a) increase to 1.3
Solution
Chemistry ma’am · 3 steps
Doubling the volume halves [H+], and halving the concentration adds log 2 to the pH.
- Start concentration. HCl is a strong acid, so it ionises completely. pH 1.0 means [H+] = 10⁻¹ = 0.1 mol L⁻¹.
- Dilute. Adding an equal volume of water doubles the volume while the moles of H⁺ stay the same. So [H+] becomes 0.1/2 = 0.05 mol L⁻¹.
pH = -log(10⁻¹/2) = 1 + log 2 = 1 + 0.30 = 1.3
- So. The solution becomes less acidic and its pH increases to 1.3, which is option (a).
The idea to keep. pH is a log scale. Diluting by a factor n raises the pH of a strong acid by log n, not by n times. Two-fold dilution adds 0.3, ten-fold adds 1.
Answer from the NTA final answer key. Solution by PrepWiser.