If you add 5.0 mL of 0.50 M HCl solution to 20.0 mL to Buffer C, what is the pH of the buffer? (where buffer C is 8.203 g sodium acetate with 100.0 mL of 1.0 M acetic acid)
mols NaAc = 8.203/60 = about 0.14 but you need to be more accurate.
M NaAc = 0.14/0.1L = about 1.4M
millimoles NaAc = 20 mL x 1.4 = 28
mmols HAc = 20 mL x 1.OM = 20
mmols HCl = 5 mL x 0.50M = 02.5
..........Ac^- + H^+ ==> HAc
initial...28.....0.......20
add..............2.5.........
change....-2.5..-2.5......+2.5
equil....25.5.....0........2.5
Recalculate the numbers to be more accuratae, substitute those numbers into the Hendeson-Hasselbalch equation and solve for the pH.
To find the pH of the buffer solution after adding HCl, we need to calculate the concentrations of the conjugate acid-base pair in the buffer solution.
1. Calculate the moles of sodium acetate (CH3COONa):
- We have the mass of sodium acetate, which is 8.203 g.
- The molar mass of sodium acetate (CH3COONa) is 82.03 g/mol.
- Using the formula: moles = mass / molar mass, we can find the moles of sodium acetate.
2. Calculate the moles of acetic acid (CH3COOH):
- We have the volume of acetic acid, which is 100.0 mL.
- Convert the volume to liters by dividing it by 1000 (since 1 L = 1000 mL).
- The concentration of acetic acid is 1.0 M, which means 1 mole of acetic acid is present in 1 L of solution.
- So, the moles of acetic acid can be calculated using the formula: moles = volume (in L) x concentration.
3. Determine which component of the buffer solution, acetic acid or sodium acetate, will react with the added HCl:
- HCl is a strong acid, which means it will completely dissociate in water.
- HCl will react with the acetate ion (CH3COO-) to form acetic acid (CH3COOH).
4. Calculate the moles of HCl:
- The volume of HCl solution added is 5.0 mL, which can be converted to liters by dividing by 1000.
- The concentration of HCl is 0.50 M, so we can calculate the moles using the formula: moles = volume (in L) x concentration.
5. Determine the limiting reagent:
- A buffer solution consists of a weak acid and its conjugate base.
- The limiting reagent will be the component that is completely consumed in the reaction with HCl.
- Compare the moles of HCl with the moles of acetate ion in the buffer solution to determine the limiting reagent.
6. Calculate the resulting moles of acetic acid and acetate ion remaining:
- Subtract the moles of HCl that reacted from the initial moles of acetic acid and acetate ion in the buffer solution.
- The remaining moles will determine the concentrations of acetic acid and acetate ion.
7. Calculate the concentrations of acetic acid and acetate ion:
- Use the formula: concentration = moles / volume (in L).
8. Calculate the pH of the buffer solution using the Henderson-Hasselbalch equation:
- pH = pKa + log ([A-] / [HA])
- The pKa value of acetic acid is known to be 4.76.
- [A-] is the concentration of acetate ion.
- [HA] is the concentration of acetic acid.
By following these steps and performing the necessary calculations, the pH of the buffer solution after adding HCl can be determined.