# Buffers, Hydrolysis and Titration

Chemistry I · Energy, Equilibrium and Electrochemistry · https://tryals.app/learn/chemistry-i/buffers-hydrolysis-and-titration

## Solutions That Push Back

A **buffer** resists pH change on addition of acid or base. It contains a weak acid together with its conjugate base in comparable amounts, so that added $\mathrm{H^+}$ is mopped up by the base and added $\mathrm{OH^-}$ by the acid. The **Henderson-Hasselbalch equation** gives the pH:

$$\mathrm{pH} = \mathrm{p}K_a + \log\frac{[\mathrm{A^-}]}{[\mathrm{HA}]}$$

Two consequences follow immediately. When the two are equal the log term vanishes and $\mathrm{pH} = \mathrm{p}K_a$, so a buffer is chosen by picking an acid whose $\mathrm{p}K_a$ is near the target pH. And because only the *ratio* appears, diluting a buffer barely changes its pH, though it does reduce its **capacity**, the amount of acid or base it can absorb before failing.

**Salt hydrolysis** explains why dissolving a salt often gives a non-neutral solution. The ions inherit the strength of their parents:

| Salt from | Resulting solution |
|---|---|
| Strong acid + strong base | Neutral |
| Weak acid + strong base | Basic |
| Strong acid + weak base | Acidic |

Sodium acetate is basic because acetate is the conjugate of a weak acid and takes protons from water; ammonium chloride is acidic for the mirror reason.

A **titration** adds a known reagent until the reaction is complete. The **equivalence point** is where stoichiometrically equal amounts have been mixed; the **endpoint** is where the indicator changes. These are ideally, but not automatically, the same, and choosing an indicator whose range brackets the equivalence pH is the analyst's job.

The equivalence pH is not always 7. Strong acid with strong base gives 7; **weak acid with strong base gives above 7**, because the conjugate base remains in solution. Halfway to equivalence, exactly half the weak acid is neutralised, the buffer ratio is 1, and $\mathrm{pH} = \mathrm{p}K_a$, the standard way of measuring $K_a$ from a curve.

> **Common pitfall:** assuming every equivalence point is at pH 7. Only the strong-strong case is neutral. Titrating acetic acid with sodium hydroxide reaches equivalence near pH 8.7, so phenolphthalein is appropriate and methyl orange would change far too early.

## Practice questions

8 of this lesson's 12 practice questions, with answers. The full set is in the app.

### 1. A buffer contains equal concentrations of a weak acid and its conjugate base. The acid has $\mathrm{p}K_a = 4.75$. Compute the pH of the buffer.

**Answer:** 4.75 (within ±0.05)

**Why:** With $[\mathrm{A^-}] = [\mathrm{HA}]$ the ratio is 1 and $\log 1 = 0$, so $\mathrm{pH} = \mathrm{p}K_a = 4.75$. This is why a buffer is chosen by matching its $\mathrm{p}K_a$ to the pH you want to hold.

Page: https://tryals.app/practice/chemistry-i/buffers-hydrolysis-and-titration/a-buffer-contains-equal-concentrations-of-a-weak-acid-and-its

### 2. Complete the description of how a buffer behaves.

**Answer:** A buffer contains a weak acid together with its **conjugate base**, and its pH equals the **pKa** when the two are present in equal amounts. Diluting the buffer leaves the pH almost **unchanged** but reduces its **capacity**.

**Why:** Only the ratio of base to acid enters the equation, so dilution leaves the pH nearly untouched. What dilution does destroy is capacity, there is simply less material available to absorb added acid or base.

Page: https://tryals.app/practice/chemistry-i/buffers-hydrolysis-and-titration/complete-the-description-of-how-a-buffer-behaves

### 3. Why does the equivalence point of a weak acid titrated with a strong base lie above pH 7?

A. Excess strong base remains unreacted in solution at equivalence
B. The conjugate base of the weak acid remains in solution and hydrolyses
C. The weak acid continuously releases hydroxide ions during neutralisation
D. The indicator selected for the titration shifts the measured equivalence pH

**Answer:** B. The conjugate base of the weak acid remains in solution and hydrolyses

**Why:** At equivalence the acid is entirely converted to its conjugate base, which is a genuine weak base and hydrolyses water to give hydroxide. There is no excess titrant at that point, the basicity comes from the salt itself.

Page: https://tryals.app/practice/chemistry-i/buffers-hydrolysis-and-titration/why-does-the-equivalence-point-of-a-weak-acid-titrated-with-a-strong

### 4. Diluting an equimolar buffer ten-fold leaves its pH virtually unchanged, yet it compromises its protective function. What accounts for this apparent contradiction?

A. The dilution shifts the pKa value whilst fixing the ion ratio
B. The acid and conjugate base dilute at unequal equilibrium rates
C. The ratio is fixed, but the total moles of buffer pair decrease
D. The capacity is unchanged, but the target pH drifts sharply

**Answer:** C. The ratio is fixed, but the total moles of buffer pair decrease

**Why:** Dilution scales both components equally, preserving the logarithmic ratio that dictates pH, but reduces the absolute reservoir of ions available to neutralise incoming stress. A system's set point is distinct from its operational endurance.

Page: https://tryals.app/practice/chemistry-i/buffers-hydrolysis-and-titration/diluting-an-equimolar-buffer-ten-fold-leaves-its-ph-virtually

### 5. Match each titration pairing to the pH at its equivalence point.

**Answer:**

- Strong acid with strong base → Exactly 7
- Weak acid with strong base → Above 7
- Strong acid with weak base → Below 7
- Half-equivalence of a weak acid → Equal to the acid pKa

**Why:** Only strong-with-strong leaves ions that do not hydrolyse, giving a neutral 7. A weak parent on either side leaves a hydrolysing conjugate that pushes the equivalence pH away from neutral.

Page: https://tryals.app/practice/chemistry-i/buffers-hydrolysis-and-titration/match-each-titration-pairing-to-the-ph-at-its-equivalence-point

### 6. Sort each action by its effect on a buffer solution.

**Answer:**

- Barely changes the pH: Adding a small amount of strong acid, Adding a small amount of strong base, Diluting the buffer with water
- Destroys the buffering ability: Adding enough strong base to consume all the weak acid

**Why:** Small additions are absorbed by the conjugate pair and dilution preserves the ratio. Once one component is fully consumed there is nothing left to absorb further additions, and the pH then moves sharply.

Page: https://tryals.app/practice/chemistry-i/buffers-hydrolysis-and-titration/sort-each-action-by-its-effect-on-a-buffer-solution

### 7. A buffer is made from an acid of $\mathrm{p}K_a = 4.75$ with a conjugate base to acid ratio of 10 to 1. Set the pH of this buffer.

**Answer:** 5.75 (within ±0.2)

**Why:** $\mathrm{pH} = \mathrm{p}K_a + \log 10 = 4.75 + 1 = 5.75$. A tenfold excess of conjugate base buys exactly one pH unit, and pushing much beyond that leaves the buffer with very little capacity against added base.

Page: https://tryals.app/practice/chemistry-i/buffers-hydrolysis-and-titration/a-buffer-is-made-from-an-acid-of-pka-4-75-with-a-conjugate-base-to

### 8. Which are true of an effective buffer solution?

A. It contains a weak acid and its conjugate base in comparable amounts
B. Its pH is close to the pKa of the weak acid
C. Its pH is almost unchanged by moderate dilution
D. It can absorb unlimited amounts of added strong acid

**Answer:** A. It contains a weak acid and its conjugate base in comparable amounts; B. Its pH is close to the pKa of the weak acid; C. Its pH is almost unchanged by moderate dilution

**Why:** A buffer needs both partners in comparable amounts, works best near the $\mathrm{p}K_a$, and survives dilution because only the ratio matters. Its capacity is strictly finite, and once exhausted the pH moves abruptly.

Page: https://tryals.app/practice/chemistry-i/buffers-hydrolysis-and-titration/which-are-true-of-an-effective-buffer-solution