Here are the essential concepts you must grasp in order to answer the question correctly.
pKa and Acid-Base Indicators
The pKa value of an acid is the pH at which half of the acid is dissociated into its conjugate base. Methyl red, with a pKa of 5.0, serves as an acid-base indicator, changing color based on the pH of the solution. Below its pKa, it appears red (acidic form), and above it, it turns yellow (basic form). This property is crucial for determining the color of the solution in different pH environments.
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Strong Acids and pH Calculation
Strong acids, like HCl, completely dissociate in solution, meaning that the concentration of hydrogen ions [H⁺] equals the concentration of the acid. In a 0.100 M HCl solution, the pH can be calculated using the formula pH = -log[H⁺], resulting in a pH of 1. This low pH indicates that the solution will appear red when methyl red is added, as it is below the pKa of 5.0.
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pH of Strong Acids and Bases
Titration and Color Change Range
During a titration, as a strong base like NaOH is added to a strong acid like HCl, the pH of the solution gradually increases. The color change of methyl red occurs around its pKa, specifically between pH 4.4 (red) and pH 6.2 (yellow). Therefore, as NaOH is added, the indicator will transition from red to yellow as the pH crosses this range, indicating the point at which the solution becomes neutral to slightly basic.
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