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Ch.17 - Applications of Aqueous Equilibria
McMurry - Chemistry 8th Edition
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Tutti i libri di testoMcMurry 8th EditionCh.17 - Applications of Aqueous EquilibriaProblema 75
Capitolo 17, Problema 75

In what volume ratio should you mix 1.0 M solutions of NH4Cl and NH3 to produce a buffer solution having pH = 9.80?

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Step 1: Identify the buffer system. In this case, the buffer system is NH4Cl/NH3. NH4Cl is the acidic component (NH4+) and NH3 is the basic component.
Step 2: Use the Henderson-Hasselbalch equation, which is pH = pKa + log([A-]/[HA]), where [A-] is the concentration of the base (NH3) and [HA] is the concentration of the acid (NH4+). In this case, we are looking for the ratio [A-]/[HA], so we can rearrange the equation to [A-]/[HA] = 10^(pH - pKa).
Step 3: Find the pKa value. The Ka of NH4+ is 5.6 x 10^-10, so the pKa is -log(5.6 x 10^-10).
Step 4: Substitute the given pH and the calculated pKa into the rearranged Henderson-Hasselbalch equation to find the ratio [A-]/[HA].
Step 5: The ratio [A-]/[HA] is the volume ratio of NH3 to NH4Cl. Therefore, to prepare the buffer solution, you should mix the 1.0 M solutions of NH4Cl and NH3 in this volume ratio.

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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, NH4Cl acts as the weak acid and NH3 as the weak base, allowing the solution to maintain a stable pH.
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Buffer Solutions

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation relates the pH of a buffer solution to the concentration of its acid and base components. It is expressed as pH = pKa + log([A-]/[HA]), where [A-] is the concentration of the base and [HA] is the concentration of the acid. This equation is essential for calculating the required volume ratio of NH4Cl and NH3 to achieve the desired pH.
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Henderson-Hasselbalch Equation

pKa and pKb Values

The pKa and pKb values are measures of the strength of an acid and a base, respectively. The pKa is the negative logarithm of the acid dissociation constant (Ka), while pKb is the negative logarithm of the base dissociation constant (Kb). For the NH4+/NH3 system, knowing the pKa allows for the calculation of the necessary concentrations to achieve a specific pH, as pKa and pKb are related through the equation pKa + pKb = 14.
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