Give a recipe for preparing a CH3CO2H-CH3CO2Na buffer solution that has pH = 4.44.

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Step 1: The first step in preparing a buffer solution is to identify the acid and its conjugate base. In this case, the acid is CH3CO2H (acetic acid) and the conjugate base is CH3CO2Na (sodium acetate).

Step 2: Use the Henderson-Hasselbalch equation, pH = pKa + log([A-]/[HA]), where pH is the desired pH of the buffer, pKa is the acid dissociation constant of the weak acid, [A-] is the molar concentration of the conjugate base, and [HA] is the molar concentration of the weak acid.

Step 3: The pKa of acetic acid is approximately 4.76. Substitute the given pH and the pKa into the Henderson-Hasselbalch equation: 4.44 = 4.76 + log([CH3CO2Na]/[CH3CO2H]).

Step 4: Solve the equation for the ratio [CH3CO2Na]/[CH3CO2H]. This will give you the ratio of the amounts of sodium acetate to acetic acid needed in the buffer solution.

Step 5: Finally, decide on a practical total buffer concentration. Then, use the ratio from step 4 to determine the amounts of sodium acetate and acetic acid to add to the solution. Remember to adjust the pH if necessary by adding more acid or base.

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주요 개념

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Buffer Solutions

Buffer solutions are mixtures that resist changes in pH when small amounts of acid or base are added. They typically consist of a weak acid and its conjugate base, or a weak base and its conjugate acid. In this case, acetic acid (CH3CO2H) and sodium acetate (CH3CO2Na) form a buffer system that can maintain a stable pH around 4.44.

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation is a mathematical formula used to calculate the pH of a buffer solution. It is expressed as pH = pKa + log([A-]/[HA]), where pKa is the negative logarithm of the acid dissociation constant, [A-] is the concentration of the conjugate base, and [HA] is the concentration of the weak acid. This equation is essential for determining the appropriate ratio of acetic acid to sodium acetate needed to achieve the desired pH.

pKa of Acetic Acid

The pKa of acetic acid is a critical value that indicates its strength as a weak acid. For acetic acid, the pKa is approximately 4.76. This value is used in the Henderson-Hasselbalch equation to calculate the necessary concentrations of acetic acid and sodium acetate to create a buffer solution with a specific pH, such as 4.44.