A solution is made by mixing 50.0 mL of 0.85 M CH3COOH with 25.0 mL of 1.7 M NaOH. Is this solution a buffer?

Given
𝐾
𝑎
(
𝐶
𝐻
3
𝐶
𝑂
𝑂
𝐻
)
=
1.8
×
10
−
5

Question

A solution is made by mixing 50.0 mL of 0.85 M CH3COOH with 25.0 mL of 1.7 M NaOH. Is this solution a buffer?

Given 
𝐾
𝑎
(
𝐶
𝐻
3
𝐶
𝑂
𝑂
𝐻
)
=
1.8
×
10
−
5

BlackTom AI · Accepted Answer

A buffer requires appreciable amounts of a weak acid and its conjugate base present together so that addition of small amounts of acid or base is resisted. Here neutralization stoichiometry determines whether both species remain.

1) Calculate moles of reactants
- Moles CH3COOH = 0.0500 L × 0.85 M = 0.04250 mol
- Moles NaOH = 0.0250 L × 1.7 M = 0.04250 mol

Because the moles are equal, OH− will completely neutralize the acetic acid:
CH3COOH + OH− → CH3COO− + H2O
All 0.04250 mol CH3COOH is converted to 0.04250 mol CH3COO−; effectively no CH3COOH remains.

2) Concentration after mixing
- Total volume = 50.0 mL + 25.0 mL = 75.0 mL = 0.0750 L
- [CH3COO−] = 0.04250 mol / 0.0750 L = 0.5667 M
- [CH3COOH] ≈ 0

3) Assess buffer status and compute pH for completeness
- A buffer cannot exist if one component of the conjugate pair is essentially zero; the Henderson–Hasselbalch equation would be undefined (log([base]/[acid]) with [acid]=0).
- The solution is instead a salt of a weak acid (acetate). Acetate hydrolyzes: CH3COO− + H2O ⇌ CH3COOH + OH−.
- Use Ka(acetic) = 1.8×10−5; Kb = Kw/Ka = 1.00×10−14 / 1.8×10−5 = 5.56×10−10.
- For C = 0.5667 M, [OH−] ≈ sqrt(Kb × C) = sqrt(5.56×10−10 × 0.5667) = sqrt(3.148×10−10) = 1.77×10−5 M.
- pOH = −log(1.77×10−5) ≈ 4.75 → pH = 14.00 − 4.75 = 9.25 (basic, but not a buffer).

4) Why the alternative ("Yes") is incorrect
- Saying the mixture is a buffer would require appreciable amounts of both CH3COOH and CH3COO−. Equal stoichiometric moles of strong base and weak acid consume the acid completely, leaving essentially only acetate; therefore the conjugate acid is absent and buffering capacity is lost.

Conclusion: The solution is not a buffer. This confirms the provided answer: No.

A solution is made by mixing 50.0 mL of 0.85 M CH3COOH with 25.0 mL of 1.7 M NaOH. Is this solution a buffer? Given 𝐾 𝑎 ( 𝐶 𝐻 3 𝐶 𝑂 𝑂 𝐻 ) = 1.8 × 10 − 5Single choice

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