Refer back to equation (2), and consider carefully whether you could simply have used the pH value of the solution at the very beginning of the titration, before any base had been added, to determine either or both the molar mass and the Ka value. Which of the following is the best conclusion? The initial solution pH depends only on the concentration of the weak acid, so measuring the initial pH is sufficient to determine the concentration. The Ka can then be calculated. The initial solution pH depends only on the Ka of the weak acid, so measuring the initial pH is sufficient to determine the Ka. The concentration can then be calculated. The initial solution pH depends on the both the Ka and the concentration of the weak acid, so measuring the initial pH is insufficient to determine both these unknowns.

Question

Chemistry

Elisa

13 August, 18:44

Refer back to equation (2), and consider carefully whether you could simply have used the pH value of the solution at the very beginning of the titration, before any base had been added, to determine either or both the molar mass and the Ka value. Which of the following is the best conclusion? The initial solution pH depends only on the concentration of the weak acid, so measuring the initial pH is sufficient to determine the concentration. The Ka can then be calculated. The initial solution pH depends only on the Ka of the weak acid, so measuring the initial pH is sufficient to determine the Ka. The concentration can then be calculated. The initial solution pH depends on the both the Ka and the concentration of the weak acid, so measuring the initial pH is insufficient to determine both these unknowns.

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Answers (1)

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Sarge · Answer 1

The initial solution pH depends on the both the Ka and the concentration of the weak acid, so measuring the initial pH is insufficient to determine both these unknowns.

Explanation:

The second equation under reference is

Kₐ = [H₃O⁺] [A-] / [HA]

where Kₐ is the dissociation constant of the acid

[A-] is the concentration of its conjugate base

[HA] is the initial concentration of the acid

Definitions

An acid is a chemical substance that produces hydroxonium [H₃0⁺] in aqueous solution

The pH of an acid is a measure of the strength of the acid and is expressed as the negative logarithm of the concentration of hydrogen ions present in solution.

pH = - log₁₀ [H₃O⁺].

If the pH is known then the hydrogen ion concentration may be calculated and vice versa.

Kₐ is the dissociation constant and is a measure of how much of the acid dissociates in aqueous solution to produce hydroxonium ions.

Kₐ = [H₃O⁺] [A-] / [HA]

The relationship between pH, pKₐ and the concentration of the acids is given by

pH = pkₐ+log ([A-]/[HA])

where [HA] is the initial concentration of the acid

[A-] is the concentration of the conjugate base

hence the initial pH of the acid is not sufficient on its own as it depends on the value of the dissociation constant Kₐ as well as the concentration of the conjugate base to calculate both unknowns.