Say, for example, that you had prepared a Buffer C, in which you mixed 8.203 g of sodium acetate, NaC2H3O2, with 100.0 mL of 1.0 M acetic acid.
a. What would be the initial pH of Buffer C? HOW???????
To determine the initial pH of the buffer solution, you need to calculate the concentrations of the conjugate acid (acetic acid, HC2H3O2) and the conjugate base (sodium acetate, NaC2H3O2) present in the buffer solution. Then, you can apply the Henderson-Hasselbalch equation, which relates the pH of a buffer solution to the concentrations of its acid and base components.
Here are the steps to calculate the initial pH of Buffer C:
1. Calculate the concentration of acetic acid (HC2H3O2):
- Given: Volume of acetic acid = 100.0 mL = 0.100 L
- Given: Concentration of acetic acid = 1.0 M
- Concentration (Molarity) = moles/volume
- Moles of acetic acid = concentration x volume
- Moles of acetic acid = 1.0 M x 0.100 L = 0.100 moles
2. Calculate the concentration of sodium acetate (NaC2H3O2):
- Given: Mass of sodium acetate = 8.203 g
- Given: Molar mass of sodium acetate = 82.03 g/mol
- Moles of sodium acetate = mass/molar mass
- Moles of sodium acetate = 8.203 g / 82.03 g/mol = 0.100 moles
Note: Sodium acetate dissociates completely in water, so the concentration of sodium acetate is equal to the moles calculated.
3. Calculate the concentration of acetic acid and its conjugate base:
- Total volume of the solution = volume of acetic acid + volume of sodium acetate = 0.100 L + 100.0 mL = 0.200 L
- Concentration of acetic acid (CHC2H3O2) = moles/volume = 0.100 moles / 0.200 L = 0.500 M (Since NaC2H3O2 and HC2H3O2 have equal moles)
- Concentration of acetate ion (C2H3O2-) = moles/volume = 0.100 moles / 0.200 L = 0.500 M (Since NaC2H3O2 and HC2H3O2 have equal moles)
4. Apply the Henderson-Hasselbalch equation:
- Henderson-Hasselbalch equation: pH = pKa + log([conjugate base]/[acid])
- The pKa value of acetic acid is 4.76 (can be found in a reference source).
- [conjugate base] = 0.500 M
- [acid] = 0.500 M
- pH = 4.76 + log(0.500/0.500) = 4.76 + log(1) = 4.76
So, the initial pH of Buffer C would be approximately 4.76.