Lesson 10 of 12 · 7 min
Buffer solutions
NCERT § "Buffer Solutions"
A litre of pure water drops from pH 7 to pH 2 when 0.01 mol HCl goes in. The same acid in a litre of our acetic acid and acetate mixture moves the pH by less than 0.1. What is soaking up the acid?
The lesson in notes
In short
A buffer resists change in pH when small amounts of acid or base are added or when it is diluted.
An acidic buffer is a weak acid with its salt of a strong base (acetic acid and sodium acetate); a basic buffer is a weak base with its salt of a strong acid (ammonia and ammonium chloride).
pH of an acidic buffer: pH = pKa + log([conjugate base]/[acid]) (Henderson-Hasselbalch equation).
pOH of a basic buffer: pOH = pKb + log([conjugate acid]/[base]), and pH = 14 − pOH at 298 K.
When the acid and its conjugate base are present at equal concentrations, pH = pKa.
A buffer can also be made by partly neutralising a weak base with a strong acid: mixing 3 mmol of NH₃ with 1 mmol of HCl leaves 2 mmol NH₃ and 1 mmol NH₄⁺, a basic buffer.
Within a buffer only the ratio of amounts matters, so millimoles can be used directly without dividing by the total volume.
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Buffer pH and worked problems
The Organic Chemistry Tutor · English · Lecture · Open on YouTube