The dissociation constant of formic acid is 0.00024. The...

The dissociation constant of formic acid is $0.00024$. The hydrogen ion concentration in $0.002 \mathrm{M}-\mathrm{HCOOH}$ solution is nearly
(a) $6.93 \times 10^{-4} \mathrm{M}$
(b) $4.8 \times 10^{-7} \mathrm{M}$
(c) $5.8 \times 10^{-4} \mathrm{M}$
(d) $1.4 \times 10^{-4} \mathrm{M}$

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Key Concepts

Weak Acid Equilibrium

In the dissociation of a weak acid, only a small fraction of the acid molecules ionize, leading to a dynamic equilibrium between the undissociated acid and its ions. The equilibrium expression for this process relates the concentrations of the hydrogen ion, the conjugate base, and the un-ionized acid, and is fundamental in calculating the pH of weak acid solutions.

Acid Dissociation Constant (Ka)

The acid dissociation constant (Ka) is a quantitative measure of the strength of an acid in solution. It is defined as the equilibrium constant for the dissociation of a weak acid into its conjugate base and a hydrogen ion, and it indicates the extent to which the acid donates protons in solution.

Equilibrium Approximations

When dealing with weak acid equilibria, approximations are often employed to simplify calculations. Given that the degree of ionization is small relative to the initial concentration, it can be assumed that the change in concentration is negligible, allowing for algebraic simplifications such as using the square root of the product of the initial concentration and the acid dissociation constant to estimate the hydrogen ion concentration.

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