Question

What is the ph of a buffer system prepared by dissolving 10.70 grams of nh₄cl and 25.00 ml of 12 mol l⁻¹ nh₃ in enough water to make 1.000 l of solution? kb = 1.80 × 10-5 for nh₃.

1)9.43
2)9.08
3)9.26
4)11.32

Answer 1

The correct answer is that the pH of the buffer solution is approximately 11.03. To find the pH of the given buffer solution, calculate the moles of NH4Cl and NH3, use the pKa value for NH4+, which is derived from the Kb of NH3, and apply the Henderson-Hasselbalch equation. T

Step-by-step explanation:

The pH of a buffer solution can be calculated using the Henderson-Hasselbalch equation. The Henderson-Hasselbalch equation is pH = pKa + log ([conjugate base]/[acid]). In this case, NH4Cl is the acid and NH3 is the conjugate base. The pKa for NH4+ is 9.25 (calculated from the Kb value for NH3), so the pH can be calculated as:

pH = 9.25 + log ([NH3]/[NH4Cl])

First, we need to calculate the concentration of NH4Cl and NH3:

10.70 grams of NH4Cl is dissolved in enough water to make 1.000 L of solution. The molar mass of NH4Cl is 53.49 g/mol, so the number of moles of NH4Cl is calculated as 10.70 g/53.49 g/mol = 0.200 mol. Therefore, the concentration of NH4Cl is 0.200 mM.

25.00 ml of 12 mol/L NH3 is also dissolved in enough water to make 1.000 L of solution. The concentration of NH3 is equal to the given concentration, which is 12 mol/L.

Now we can substitute the values into the Henderson-Hasselbalch equation:

pH = 9.25 + log (12/0.200)

pH = 9.25 + log (60)

pH = 9.25 + 1.778

pH = 11.03

So, the pH of the buffer solution is approximately 11.03.