Biology Biomolecules SZABMU 2024
PMDC Verified Question 2 of 142
At 25 degrees Celsius, the concentration of each of \(H^+\) and \(OH^-\) ions in pure water is about ____ moles per liter.
A
\(10^{-6}\)
B
\(10^{-7}\)
C
\(10^{-12}\)
D
\(10^{-14}\)
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option B: \(10^{-7}\)
Concept:

The autoionization of water perfectly defines the neutral point of the pH scale.

Solution:

  • Water spontaneously, but rarely, dissociates into ions: $$ H_2O \rightleftharpoons H^+ + OH^- $$


  • At standard room temperature (25°C), this dissociation reaches an exact thermodynamic equilibrium.


  • Because one molecule of water yields exactly one \(H^+\) and one \(OH^-\), their concentrations must be perfectly equal to maintain neutrality.


  • The measured concentration of each specific ion at this temperature is precisely \(10^{-7}\) moles per liter (which is why a neutral pH is mathematically 7: \(-log(10^{-7}) = 7\)).


Why other options are incorrect:

  • \(10^{-14}\): This is the ion product constant (\(K_w = [H^+][OH^-]\)), not the individual concentration of a single ion.


  • \(10^{-6} / 10^{-12}\): Mathematically incorrect for neutral water at standard conditions.

Quality & Fidelity Assurance: Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.

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