# Further chemical equilibria

> CIE A-Level Chemistry · 9701
> Source: https://www.owlsprep.com/study/cie-9701-u18-overview/
> Weight: n/a

This unit extends core dynamic equilibrium principles to key practical chemistry systems: acid-base reactions, buffers, sparingly soluble salts, and titrations. You will learn calculations and concepts critical for both exam and experimental work.

**Prerequisites:** [Basic chemical equilibria, including Le Chatelier's principle and $K_c$ calculations](https://www.owlsprep.com/study/cie-9701-u10-equilibria-overview/)

## Learning objectives

- Apply core equilibrium principles to acid-base systems and calculate pH for all common solution types
- Explain the mechanism of buffer action and perform accurate pH calculations for buffer solutions
- Use solubility product ($K_{sp}$) to calculate solubility and predict precipitation of sparingly soluble salts
- Interpret pH titration curves for different acid-base combinations and select appropriate indicators

## Unit at a Glance

This unit follows a logical learning arc that builds from foundational acid-base concepts to applied experimental contexts. We start with core definitions and pH calculations for acids and bases, then move to buffer solutions which are essential for maintaining stable pH in biological and industrial systems. Next, we cover solubility product, a key equilibrium concept for sparingly soluble ionic compounds. Finally, we connect all prior concepts to interpreting titration curves and selecting indicators for quantitative analysis.

This unit is split into 4 connected sub-topics:
- [Acid-base equilibria](https://www.owlsprep.com/study/cie-9701-u18-acid-base-equilibria/) — Covers Bronsted-Lowry theory, $K_a$, $pK_a$, $K_w$, and pH calculations for strong/weak acids and bases.
- [Buffer solutions](https://www.owlsprep.com/study/cie-9701-u18-buffer-solutions/) — Explains buffer action, buffer preparation, and pH calculations for acidic and basic buffers.
- [Solubility product](https://www.owlsprep.com/study/cie-9701-u18-solubility-product/) — Introduces $K_{sp}$, the common ion effect, and using $K_{sp}$ to predict precipitation reactions.
- [pH titration curves and indicators](https://www.owlsprep.com/study/cie-9701-u18-ph-titration-curves-and-indicators/) — Covers interpretation of titration curves for different acid-base combinations and indicator selection rules.

## Common pitfalls

- **Wrong:** Confusing $K_a$ with $K_{sp}$ or mixing up solubility and solubility product values.
  - Why it fails: These are equilibrium constants for different types of systems, so mixing them leads to incorrect calculations.
  - Correct: Always confirm the type of system you are working with and recall the definition of the equilibrium constant before starting calculations.
- **Wrong:** Assuming all buffers are made from a weak acid and its conjugate salt.
  - Why it fails: Basic buffers (weak base + conjugate acid) are also commonly tested, requiring slightly different calculation steps.
  - Correct: Identify whether you are working with an acidic or basic buffer first before applying the pH formula.
- **Wrong:** Forgetting to adjust concentrations for dilution when mixing solutions for pH calculations.
  - Why it fails: Dilution changes the concentration of all species, which alters the final calculated pH.
  - Correct: Always calculate new concentrations after mixing before proceeding with any equilibrium calculation.

## Cheatsheet

| Concept | Key Formula/Relationship |
| --- | --- |
| Ionic product of water | $K_w = [H^+][OH^-] = 1.00 \times 10^{-14}$ at 25°C |
| pH / pOH relationship | $pH + pOH = 14$ at 25°C |
| Acid dissociation constant | $K_a = \frac{[H^+][A^-]}{[HA]}$; $pK_a = -\log_{10} K_a$ |
| Buffer pH (Henderson-Hasselbalch) | $pH = pK_a + \log_{10} \frac{[A^-]}{[HA]}$ |
| Solubility product for $M_aX_b$ | $K_{sp} = [M^{n+}]^a [X^{m-}]^b$ |
| Indicator end point range | $pH = pK_a(indicator) \pm 1$ |
| $K_{sp}$ for $AB$-type salt | $K_{sp} = s^2$, where $s$ = molar solubility |

## What's next

Begin this unit by mastering the fundamentals of acid-base equilibria, which forms the foundation for all other topics in this unit. Once you complete all four sub-topics here, you can move on to the next unit on electrochemistry, which builds on many of the equilibrium concepts you will learn in this unit.

- [Acid-base equilibria](https://www.owlsprep.com/study/cie-9701-u18-acid-base-equilibria/)
- [Buffer solutions](https://www.owlsprep.com/study/cie-9701-u18-buffer-solutions/)
- [Solubility product](https://www.owlsprep.com/study/cie-9701-u18-solubility-product/)

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