The pH of a 0.10 M solution of propanoic acid, CH3 CH2 COOH, a weak organic acid, is measured at

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The pH of a 0.10 M solution of propanoic acid, CH3 CH2 COOH,...

The $\mathrm{pH}$ of a $0.10 \mathrm{M}$ solution of propanoic acid, $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH},$ a weak organic acid, is measured at equilibrium and found to be 2.93 at $25^{\circ} \mathrm{C}$. Calculate the $K_{\mathrm{a}}$ of propanoic acid.

Key Concepts

Acid-Base Equilibrium

Acid-base equilibrium refers to the dynamic balance established in a solution where a weak acid partially dissociates into its ions. This equilibrium is characterized by the rates of the forward (dissociation) reaction and the reverse (recombination) reaction being equal. In such systems, equilibrium constants like the acid dissociation constant (Ka) can be used to quantify the position of the equilibrium and determine the extent to which the acid ionizes in water.

Acid Dissociation Constant (Ka)

The acid dissociation constant, Ka, is a quantitative measure of the strength of a weak acid in solution. It is defined as the equilibrium concentration ratio of the products (the hydrogen ion and the conjugate base) to the reactant (the undissociated acid). A larger Ka signifies a stronger acid that dissociates more completely, while a smaller Ka indicates a weaker acid that only partially ionizes.

pH and Hydrogen Ion Concentration

pH is a logarithmic scale used to express the acidity or basicity of a solution, defined as the negative logarithm of the hydrogen ion concentration. This relationship allows one to easily convert between the measurable pH value and the actual concentration of hydrogen ions in solution, which is crucial for applying equilibrium expressions in acid-base chemistry.

Weak Acids

Weak acids are substances that do not fully dissociate in aqueous solutions, meaning that at equilibrium, a significant portion of the acid remains in its undissociated state. This incomplete ionization is described by the acid dissociation constant (Ka), and it is essential to consider when analyzing the equilibrium and calculating the concentrations of all species present in the solution.

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