To calculate the change in pH when HCl or NaOH is added to a buffer solution, you need to consider the Henderson-Hasselbalch equation. This equation relates the pH of a buffer solution to the concentrations of its conjugate acid and base. The equation is:
pH = pKa + log([A-]/[HA])
Where:
pH is the acidity level of the solution
pKa is the negative logarithm of the acid dissociation constant
[A-] is the concentration of the conjugate base
[HA] is the concentration of the acid
Let's solve the two given problems one by one:
1. Change in pH when 6.00 mL of 0.100 M HCl(aq) is added:
a. First, calculate the final concentrations of NH3 and NH4Cl after adding the HCl.
- Start with the initial concentrations:
[NH3] = 0.100 M
[NH4Cl] = 0.100 M
- Determine the moles of HCl added:
Moles = concentration × volume
Moles = 0.100 M × (6.00 mL / 1000 mL)
Moles = 0.0006 mol
- Calculate the new molar concentrations:
[NH3] = [NH3]initial - moles HCl / (initial volume + added volume)
[NH3] = 0.100 M - 0.0006 mol / (100.0 mL + 6.00 mL)
[NH3] = 0.0943 M
[NH4Cl] = [NH4Cl]initial + moles HCl / (initial volume + added volume)
[NH4Cl] = 0.100 M + 0.0006 mol / (100.0 mL + 6.00 mL)
[NH4Cl] = 0.1057 M
- Now, use the Henderson-Hasselbalch equation to calculate the new pH:
pH = pKa + log([NH3] / [NH4Cl])
(pKa for NH3/NH4Cl = 9.25)
pH = 9.25 + log(0.0943 M / 0.1057 M)
pH = 9.20
Therefore, the change in pH when 6.00 mL of 0.100 M HCl(aq) is added is -0.05.
2. Change in pH when 6.00 mL of 0.100 M NaOH(aq) is added to the original buffer solution:
a. Follow the same steps as above to determine the new concentrations of NH3 and NH4Cl after adding NaOH.
- Calculate the moles of NaOH added using the concentration and volume.
- Adjust the concentrations of NH3 and NH4Cl accordingly.
b. Use the Henderson-Hasselbalch equation to calculate the new pH.
- pH = pKa + log([NH3] / [NH4Cl])
Following these steps, you can find the change in pH when 6.00 mL of 0.100 M NaOH(aq) is added to the original buffer solution.