The active ingredient in aspirin is acetyl salicylic acid with Ka=4.0 ×10^-9 . The pH of the sol

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The active ingredient in aspirin is acetyl salicylic acid...

The active ingredient in aspirin is acetyl salicylic acid with $K_{\mathrm{a}}=4.0 \times 10^{-9} .$ The pH of the solution obtained by dissolving two aspirin tablets (containing $0.36 \mathrm{~g}$ of acetyl salicylic acid in each tablet) in $250 \mathrm{ml}$ of water is $(\log 2=0.3)$ (a) $5.1$ (b) $8.9$ (c) $10.2$ (d) $5.25$

Key Concepts

pH Calculation using Logarithms

pH is the negative logarithm of the hydrogen ion concentration in a solution. Calculating the pH of a weak acid involves estimating the hydrogen ion concentration from the established equilibrium and then applying the logarithmic function. The logarithmic scale compresses the wide range of possible hydrogen ion concentrations into a manageable scale, where small changes in concentration can lead to significant changes in pH.

Molarity and Concentration Determination

Determining the molar concentration of the acid in solution is a key initial step in pH calculation. This involves converting the mass of the acid to moles and then dividing by the volume of the solution. Accurate concentration calculations ensure that the subsequent equilibrium and pH estimations are based on the correct chemical amounts.

Acid Dissociation Constant (Ka)

Ka is a quantitative measure of the strength of an acid in solution. For a weak acid, it defines the equilibrium between the undissociated acid and its ions. A small Ka value indicates that the acid does not ionize much in solution, which is key when calculating the pH of weak acid solutions.

Weak Acid Equilibrium

In a solution of a weak acid, the equilibrium established is between the undissociated acid molecules and the ions produced by its partial dissociation into H? and its conjugate base. The degree of dissociation is governed by the acid’s Ka, and understanding this concept is crucial for determining the concentration of hydrogen ions in the solution.

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