# Acid-base language and solution thinking

Physics I · Chemistry · https://tryals.app/learn/physics-i/acid-base-language-and-solution-thinking

## Acid-Base Foundations

Chemistry changes completely when you track protons. The **Brønsted-Lowry definition** cuts through the noise: an **acid** donates a proton ($H^+$), and a **base** accepts one.

Water itself participates through **autoprotolysis**, splitting into ions with a constant product at $25 ^{\circ}C$:

$K_w = [H_3O^+][OH^-] = 10^{-14}$

Acidity is measured on a logarithmic scale. The **pH scale** formula is:

$\text{pH} = -\log_{10}[H_3O^+]$

| pH Range | Classification |
| --- | --- |
| pH $< 7$ | Acidic |
| pH $= 7$ | Neutral |
| pH $> 7$ | Basic |

**Common pitfall:** Because pH is logarithmic, each unit is a factor of 10 in $[H^+]$. pH 3 is one hundred times more acidic than pH 5, not just a bit more.

## Equilibria and Titrations

For a **weak acid equilibrium**, molecules partially dissociate in water. The acid dissociation constant is:

$K_a = \frac{[A^-][H_3O^+]}{[HA]}$

To calculate buffer pH, use the **Henderson-Hasselbalch equation**:

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

**Tip:** This equation only works when the buffer ratio $[A^-]/[HA]$ stays between 0.1 and 10.

Adding strong base to a weak acid traces an **S-shaped curve** during a **titration** with two critical milestones:

| Titration Point | Key Condition |
| --- | --- |
| **Half-equivalence** | $\text{pH} = \text{p}K_a$ (max buffer capacity) |
| **Equivalence point** | Moles acid = moles base (pH $> 7$) |

**Indicator choice:** Always select a dye whose colour change matches the equivalence pH.

## Practice questions

4 of this lesson's 14 practice questions, with answers. The full set is in the app.

### 1. Lemon juice is pH 2; black coffee is pH 5. How much more acidic (higher $H^{+}$ concentration) is the lemon juice?

A. 30×, ten times three units
B. 1000×, ten per unit
C. 3×, three units difference
D. 8×, two cubed

**Answer:** B. 1000×, ten per unit

**Why:** $10^{3} = 1000\times$. Logarithmic scales compress huge ranges into small numbers, the same trick as decibels and earthquake magnitudes. Misreading pH differences as linear is off by orders of magnitude.

Page: https://tryals.app/practice/physics-i/acid-base-language-and-solution-thinking/lemon-juice-is-ph-2-black-coffee-is-ph-5-how-much-more-acidic

### 2. A buffer resists pH change when acid is added, but only up to a point. Why does a buffer eventually stop resisting the change?

A. The strength of the acid, since strong acids cannot be buffered
B. The conjugate base available to neutralise added acid
C. The temperature, above which buffers cease to function
D. The pH of the buffer, which must be close to 7 to work

**Answer:** B. The conjugate base available to neutralise added acid

**Why:** Buffer pH and buffer capacity are different quantities. The ratio of the pair sets the pH; their absolute amounts set how much insult the buffer can take.

Page: https://tryals.app/practice/physics-i/acid-base-language-and-solution-thinking/a-buffer-resists-ph-change-when-acid-is-added-but-only-up-to-a

### 3. Order the steps to identify a conjugate acid-base pair in a reaction.

**Answer:**

1. Write the proton-transfer reaction: HA + B ⇌ A⁻ + BH⁺
2. Identify the acid HA (proton donor) and its conjugate base A⁻
3. Identify the base B (proton acceptor) and its conjugate acid BH⁺
4. Verify: each conjugate pair differs by exactly one proton H⁺

**Why:** A conjugate pair always has the acid form containing one more H⁺ than its conjugate base. Ka × $Kb = Kw$ for a conjugate pair.

Page: https://tryals.app/practice/physics-i/acid-base-language-and-solution-thinking/order-the-steps-to-identify-a-conjugate-acid-base-pair-in-a-reaction

### 4. In each reaction, identify the highlighted species’ role in Brønsted–Lowry terms.

**Answer:**

- Acid (proton donor): HCl in: HCl + H₂O → Cl⁻ + H₃O⁺, H₂O in: NH₃ + H₂O → NH₄⁺ + OH⁻
- Base (proton acceptor): NH₃ in: NH₃ + H₂O → NH₄⁺ + OH⁻, H₂O in: HCl + H₂O → Cl⁻ + H₃O⁺

**Why:** Acidity is a role, not an identity: water donates a proton to ammonia but accepts one from HCl. Such amphoteric behaviour is the heart of the Brønsted–Lowry view, every acid–base reaction is just a proton handoff.

Page: https://tryals.app/practice/physics-i/acid-base-language-and-solution-thinking/in-each-reaction-identify-the-highlighted-species-role-in