Chapter 104 of 2363 min read

Ionic equilibrium: acids, bases, pH, buffers

Why this is asked: For a strong acid diluted to around 10⁻⁷–10⁻⁸ M, pH = −log(C) alone gives a value above 7 — impossible for an acid — because it ignores water's own contribution of H⁺ at that concentration. This "pH of an extremely dilute strong acid" question is a NEET standard precisely because the naive formula gives a physically impossible answer.

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Test yourself

•Phosphoric acid ionises in three steps with ionisation constant values $K_{a_{1}}$ , $K_{a_{2}}$ and $K_{a_{3}}$ respectively, while $K$ is the overall ionisation constant. Which of the following statements are true? A. $lo g K = lo g K_{a_{1}} + lo g K_{a_{2}} + lo g K_{a_{3}}$ B. $H_{3} P O_{4}$ is a stronger acid than $H_{2} P O_{4}^{-}$ and $HP O_{4}^{2 -}$ C. $K_{a_{1}} > K_{a_{2}} > K_{a_{3}}$ D. $\frac{K _{a_{2}} + K _{a_{3}}}{K _{a_{1}}} = 2$ Choose the correct answer from the options given below:

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