JEE Main 2022 · 27 July · evening · Single correct · +4 / −1

The plot of pH-metric titration of weak base NH₄ OH vs strong acid HCl looks like :

  1. (a)
    Option (a)
  2. (b)
    Option (b)
  3. (c)
    Option (c)
  4. (d)
    Option (d)
Answer and solution

Answer (a)

Solution

Chemistry ma’am · 3 steps

Acid is going into a base, so pH must fall. A weak base falls gently, with no tall vertical cliff.

  1. Direction. The flask holds NH₄OH, so the curve starts above 7 (about 11 for a 0.1 M solution). Every drop of HCl removes base, so the pH can only go down.
  2. Shape. Once acid is added, the flask holds both NH₄OH and NH₄Cl, which is a basic buffer. The pH therefore drops slowly, the jump near equivalence is short and gradual, and the equivalence point lies below 7 because NH₄⁺ hydrolyses. Past equivalence, excess HCl levels the pH off at a low value.
    OptionStarts atDirectionJumpFits weak base vs HCl?
    (a)above 7fallsgradual S-shapeyes
    (b)below 7risesgradualno, wrong direction
    (c)above 7fallstall and verticalno, that is strong base vs strong acid
    (d)below 7risestall and verticalno, wrong direction
  3. So. Only (a) starts basic, falls, and changes smoothly instead of plunging. That is the curve for NH₄OH titrated with HCl.

The idea to keep. Read a titration curve in two passes. The starting pH and direction tell you what is in the flask. The height and sharpness of the jump tell you whether a weak partner is buffering it.

Answer from the NTA final answer key. Solution by PrepWiser.

Study Ionic Equilibrium

Asked about 100 times in JEE Main since 2020. This question is from 2022.