A 1.0L buffer solution contains 0.100 mol
of HC2H3O2and 0.100 molof NaC2H3O2
. The value of Ka
for HC2H3O2 is 1.8×10−5.
Calculate the pH of the solution upon the addition of 0.015 mol NaOH to the original buffer
Calculate the pH of the solution upon the addition of 10.0 mL of 1.00 M HCl to the original buffer.
HC2H3O2 = HAc and C2H3O2^- = Ac^-
First the addition of NaOH.
..................HAc + OH^- ==> Ac^- + HOH
I.................0.1.......0.................0.1.................
add....................0.015..................................
C............-0.015 ...-0.015.............+0.015................
E..............0.085.......0...................0.115
(HAc) = 0.085/volume what ever that is. It's the same for (Ac^-) and that is (0.115/volume). Note the volumes cancel, whatever that is, so the equation looks like this. pH = pKa + log (base/acid) which is the Henderson-Hasselbach equation
pH = 4.74 + log (0.115/0.085) and solve for pH.
For part b, the addition of HCl it is done the same way as the addition of NaOH except the equation is a little different since you're adding acid and not base AND the volumes are given so you can calculate the concentration to put into the HH equation. Just remember though that the volume cancels so you really don't need to do the actual concentration calculation; i.e., you can use moles. I'll leave that calculation for you. Post your work if you get stuck. Here is the new reaction you need.
..........................Ac^- + H^+ ==> HAc
To calculate the pH of the solution upon the addition of NaOH and HCl to the original buffer, we need to consider the change in concentration of the acidic and basic components in the buffer, as well as the Ka value.
1. Calculate the moles of the acidic component (HC2H3O2) and the basic component (NaC2H3O2) in the original buffer solution:
- Moles of HC2H3O2 = 0.100 mol
- Moles of NaC2H3O2 = 0.100 mol
2. Calculate the concentration of each component in the original buffer solution:
- Concentration of HC2H3O2 = Moles of HC2H3O2 / Volume of solution (in liters)
- Concentration of NaC2H3O2 = Moles of NaC2H3O2 / Volume of solution (in liters)
3. Add the moles of NaOH and HCl to the original buffer solution:
a) Addition of NaOH:
- Moles of NaOH added = 0.015 mol
- Subtract the moles of NaOH from the moles of NaC2H3O2 in the buffer solution
b) Addition of HCl:
- Volume of HCl added = 10.0 mL = 0.010 L
- Moles of HCl added = Concentration of HCl * Volume of HCl
4. Calculate the new concentrations of HC2H3O2 and NaC2H3O2 in the buffer after adding NaOH or HCl.
a) After adding NaOH:
- Concentration of HC2H3O2 remains the same
- Concentration of NaC2H3O2 = (Moles of NaC2H3O2 - Moles of NaOH) / Volume of solution (in liters)
b) After adding HCl:
- Concentration of HC2H3O2 = Concentration of HC2H3O2 - (Moles of HCl / Volume of solution)
- Concentration of NaC2H3O2 remains the same
5. Calculate the ratio of [HC2H3O2] / [C2H3O2-]:
- [HC2H3O2] / [C2H3O2-] = Concentration of HC2H3O2 / Concentration of NaC2H3O2
6. Calculate the pH of the buffer solution using the Henderson-Hasselbalch equation:
- pH = pKa + log([HC2H3O2] / [C2H3O2-])
Let's calculate the pH after the addition of NaOH and HCl to the original buffer solution.