Understanding Solubility Product Quotient (Q) and Precipitation

Expert reviewed • 22 November 2024 • 4 minute read

Introduction

The solubility product quotient (Q) is a fundamental concept in solution chemistry that helps us predict whether precipitation will occur in ionic solutions. Understanding Q and its relationship to the solubility product constant (Ksp) is crucial for analyzing chemical equilibria in solution.

Key Concepts

Solubility Product Quotient vs. Equilibrium

The solubility product quotient Q represents the product of ion concentrations in a solution at any given moment, while Ksp represents this product specifically at equilibrium. When comparing Q and Ksp:

  • If Q<KspQ < K_{sp}: The solution is unsaturated; more solid can dissolve
  • If Q=KspQ = K_{sp}: The solution is at equilibrium
  • If Q>KspQ > K_{sp}: The solution is supersaturated; precipitation will occur

Relationship to Solubility

The value of Ksp indicates the solubility of an ionic compound:

  • High Ksp values indicate high solubility
  • Low Ksp values indicate low solubility

Calculating Q and Predicting Precipitation

Let's explore three examples of calculating Q and predicting precipitation.

Barium Sulfate Solubility:

Practice Question 1

0.100 g of BaSO₄ is added to 500.0 mL of water at 25°C. Will precipitation occur?
1. Write the dissociation equation:

BaSO4(s)⇌Ba2+(aq)+SO42−(aq)\text{BaSO}_4(s) \rightleftharpoons \text{Ba}^{2+}(aq) + \text{SO}_4^{2-}(aq)

  • Calculate moles of BaSO₄: n=0.100 g137.3+32.07+(16×4)=4.29×10−4 moln = \frac{0.100\text{ g}}{137.3 + 32.07 + (16 × 4)} = 4.29 × 10^{-4}\text{ mol}

  • Calculate ion concentrations: [Ba2+]=[SO42−]=4.29×10−4 mol0.500 L=8.57×10−4 M[\text{Ba}^{2+}] = [\text{SO}_4^{2-}] = \frac{4.29 × 10^{-4}\text{ mol}}{0.500\text{ L}} = 8.57 × 10^{-4}\text{ M}

  • Calculate Q: Q=[Ba2+][SO42−]=(8.57×10−4)2=7.34×10−7Q = [\text{Ba}^{2+}][\text{SO}_4^{2-}] = (8.57 × 10^{-4})^2 = 7.34 × 10^{-7}

  • Compare with Ksp: Ksp of BaSO4=1.08×10−10K_{sp}\text{ of BaSO}_4 = 1.08 × 10^{-10}

∴\therefore Since Q>KspQ > K_{sp}, precipitation will occur.

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Understanding Solubility Product and Equilibrium in Solutions

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