Question

How much 5.60M NaOH must be added to 410.0 mL of a buffer that is 0.0205 M acetic acid and 0.0235 M sodium acetate to raise the pH to 5.75?

Answer 1

Values:

Base: NaOH 5.60M x mL to rise pH buffer at 5.75 units

Buffer: CH₃-COOH 0.0205M / CH₃-COONa 0.0235M V_buffer=410 mL

pKa: CH₃-COOH 4.7

Process:

pH = pKa + Log₁₀[Salt]/[Acid]

pH = 4.7 + Log₁₀[0.0235M]/[0.0205M]

pH = 4.76

How much NaOH need to rise pH 4.76 to 5.75?

If: 410 mL Buffer + x mL NaOH ---------> pH=5.75; buffer aport 4.76 pH unit + 0.99 pH unit aport NaOH.

Then:

M1 x V2 = M2 x V1

-Log₁₀M₁ x V₂ = -Log₁₀M₂ x V₁

pH₁ x V₂ = pH₂ x V₁

4.76 x V₂ = 5.75 x 410 mL

V₂=(5.75 x 410 mL) / 4.76

V₂=495.27 mL is the final volume to pH rise at 4.76 to 5.75, if the initial buffer volume is 410 mL the excess volume is 85.27 mL, this volume correspond at NaOH 5.60M