Total volume = 800 mL = 0.8 L pH = pKa + log [A-] / [HA]<~~~Henderson-Hasselbalc

Question

Total volume = 800 mL = 0.8 L

pH = pKa + log [A-] / [HA]<~~~Henderson-Hasselbalch equation.

moles NaOH = 2.0 mol/L * (0.3 L) = 0.6 moles

moles CH3COOH = 2.0 mol/ L * (0.5 L) = 1 mole

[NaOH] = 0.6 moles / 0.8 L = 0.75 mol/L
[CH3COOH] = 1 mole / 0.8 L = 1.25 mol/L


pKa = -log(Ka)
pKa = -log(1.8 x 10^-5) = 4.74

pH = 4.74 + log(1.25/0.75)

pH = 4.9 <~~~Answer

I've seen this solution a few times, but cannot figure out why the equation is pH=4.74+log(1.25/.75) which puts HA/A- instead of A-/HA, as stated in the equation?

Explanation / Answer

pH = pka + log [A-]/[HA]   

always equation has to be put in this manner , reversing ratio will cause wrong answer

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