Question

What mass of NH₄Cl must be added to 0.750 L of a 0.100-M solution of NH₃ to give a buffer solution with a pH of 9.26? (Hint: Assume a negligible change in volume as the solid is added.)

a) 0.00 g
b) Between 0.00 g and 10.00 g
c) 10.00 g
d) More than 10.00 g

Answer 1

To calculate the mass of NH₄Cl that must be added to the solution, you can use the Henderson-Hasselbalch equation and the given values. The mass of NH₄Cl that must be added is approximately 0.4017 g.

Step-by-step explanation:

To calculate the mass of NH₄Cl that must be added to the solution, we can use the Henderson-Hasselbalch equation:

pH = pKa + log([A-]/[HA])

Since we want a pH of 9.26, and the pKa of NH₃/NH₄Cl is 9.25, we can rearrange the equation to solve for [A-]/[HA] and substitute the given values:

[A-]/[HA] = 10^(pH - pKa) = 10^(9.26 - 9.25) = 10

Since the concentration of NH₃ in the solution is 0.100 M, we can assume that [HA] = 0.100/10 = 0.010 M. Therefore, the concentration of [A-] must also be 0.010 M.

Now, we can calculate the moles of NH₄Cl that must be added:

Moles of NH₄Cl = concentration of [A-] in moles × volume of solution

= 0.010 M × 0.750 L = 0.0075 moles

To calculate the mass of NH₄Cl, we can use the molar mass of NH₄Cl:

Mass of NH₄Cl = moles of NH₄Cl × molar mass of NH₄Cl

= 0.0075 moles × 53.49 g/mol (molar mass of NH₄Cl)

= 0.4017 g

Therefore, the mass of NH₄Cl that must be added to the solution is approximately 0.4017 g. The correct answer is b) Between 0.00 g and 10.00 g.