A Buffer Solution and Its pH Calculation
Determine the pH for the original buffer solution
A buffer solution is prepared by combining 100 mL of concentrated ammonia (14.5 M) and 100 g of NH₄NO₃ with water to produce a total volume of 800 mL. Kb for ammonia is 1.8 x 10⁻⁵.
Final Answer:
The pH of the original buffer solution is approximately 9.64. This was calculated using the Henderson-Hasselbalch equation with the pKa of ammonia (NH₃) and the initial concentrations of NH₃ and NH₄⁺. The ammonia is a weak base, leading to a slightly basic pH.
Explanation:
To determine the pH of the buffer solution, we'll use the Henderson-Hasselbalch equation:
pH = pKa + log([A-]/[HA])
In this equation:
- pKa is the negative logarithm (base 10) of the dissociation constant (Kb) for ammonia, which is given as 1.8 x 10⁻⁵. So, pKa = -log(1.8 x 10⁻⁵) ≈ 4.74.
- [A-] is the concentration of the base (NH₃) in the buffer, which is initially 14.5 M * 0.1 L = 1.45 moles.
- [HA] is the concentration of the ammonium ion (NH₄⁺) in the buffer, which is initially 0.1 kg / 64.04274 g/mol = 1.56 moles.
Now, let's calculate the pH:
pH = 4.74 + log(1.45/1.56) ≈ 9.64
So, the pH of the original buffer solution is approximately 9.64.
Note: The mass of NH₄NO₃ was converted to moles by dividing by its molar mass (64.04274 g/mol), and the volume of concentrated ammonia was converted to moles by multiplying by its concentration (14.5 M). The Henderson-Hasselbalch equation is used to calculate the pH of a buffer solution.
How was the pH of the original buffer solution calculated? The pH of the original buffer solution was calculated using the Henderson-Hasselbalch equation, which involves the pKa value of ammonia (NH₃) and the initial concentrations of NH₃ and NH₄⁺ in the buffer solution. By plugging in the values and performing the calculations, the pH was determined to be approximately 9.64.