Rank The Following Aqueous Solutions In Order Of Electrical Conductivity

Ranking Aqueous Solutions by Electrical Conductivity: A Clear Guide

Electrical conductivity in aqueous solutions is a fundamental concept in chemistry that reveals the invisible world of ions in water. When a substance dissolves, it may break apart into charged particles called ions. These ions are the carriers of electric current through the solution. The ability of a solution to conduct electricity—its conductivity—depends entirely on the presence, concentration, and mobility of these ions. Understanding how to rank solutions from least to most conductive is crucial for predicting chemical behavior, designing industrial processes, and even interpreting medical tests like blood electrolyte panels. This guide will walk you through the scientific principles and provide a clear methodology for ranking any set of common aqueous solutions.

The Core Principle: Ions Are the Key

A solution conducts electricity only if it contains free-moving charged particles. Pure water itself is a very poor conductor because it self-ionizes to an extremely small extent, producing only a tiny concentration of H⁺ and OH⁻ ions. The magic happens when an ionic compound or a polar molecular compound dissolves and releases ions.

• Strong Electrolytes: These substances, including all soluble salts (like NaCl, KNO₃), strong acids (HCl, HNO₃, H₂SO₄), and strong bases (NaOH, KOH), dissociate completely (100%) into ions in water. A 0.1 M solution of NaCl, for instance, yields approximately 0.1 M Na⁺ ions and 0.1 M Cl⁻ ions, providing a high density of charge carriers.
• Weak Electrolytes: These substances, such as weak acids (CH₃COOH, HCN) and weak bases (NH₃), dissociate only partially (typically 1-5%) in water. The vast majority of the molecules remain intact. A 0.1 M acetic acid solution might only produce about 0.001 M H⁺ ions and 0.001 M CH₃COO⁻ ions, resulting in much lower conductivity than a strong electrolyte at the same nominal concentration.
• Nonelectrolytes: Compounds like sugar (C₁₂H₂₂O₁₁), ethanol (C₂H₅OH), and methanol dissolve in water but do not produce ions at all. Their solutions have virtually zero electrical conductivity.

Therefore, the first and most critical step in ranking conductivity is to classify each solute as a strong electrolyte, weak electrolyte, or nonelectrolyte.

Factors Influencing Conductivity Ranking

Once you know the electrolyte type, two primary factors determine the final order:

1. Concentration of Ions: For solutions of the same electrolyte type (e.g., two strong electrolytes), a higher molar concentration generally means more ions and higher conductivity. A 1.0 M HCl solution will conduct better than a 0.1 M HCl solution.
2. Nature of the Ions (Charge and Mobility):
   • Ion Charge: Ions with higher absolute charge carry more current. A solution of MgCl₂ (providing Mg²⁺ and Cl⁻ ions) will conduct better than a NaCl solution at the same molar concentration because the divalent Mg²⁺ ion contributes twice the charge per ion compared to Na⁺.
   • Ion Mobility: Smaller, less hydrated ions move faster through water under an electric field. For example, H⁺ and OH⁻ ions have exceptionally high mobility due to a unique "hopping" mechanism (the Grotthuss mechanism), making even dilute strong acids and bases surprisingly conductive. K⁺ ions are more mobile than larger Cs⁺ ions. Among common anions, OH⁻ is highly mobile, while larger ions like I⁻ are more mobile than smaller, more heavily hydrated ones like F⁻.

A Practical Ranking Methodology

To rank a given list of solutions, follow this systematic approach:

Step 1: Identify the Solute and Its Concentration. Write down the chemical formula and molarity (e.g., 0.1 M CH₃COOH, 0.2 M NaCl).

Step 2: Classify the Electrolyte. Is it strong, weak, or non? This is your primary sorting filter.

• Group 1 (Highest Conductivity): All strong electrolytes.
• Group 2 (Medium Conductivity): All weak electrolytes.
• Group 3 (Lowest/Zero Conductivity): All nonelectrolytes.

Step 3: Rank Within the Strong Electrolyte Group.

• First, compare total ion concentration. For a compound AₓBᵧ that dissociates into x cations and y anions, the total molar ion concentration is (x+y) * Molarity of the compound. A 0.1 M CaCl₂ solution (total ions = 0.3 M) will conduct better than a 0.1 M NaCl solution (total ions = 0.2 M).
• If total ion concentrations are similar, consider ion charge. A 0.1 M Al(NO₃)₃ solution (total ions = 0.4 M, with Al³⁺) will outperform a 0.1 M Na₂SO₄ solution (total ions = 0.3 M) due to the high charge on Al³⁺.
• For strong acids/bases at very low concentrations, remember the exceptional mobility of H⁺ and OH⁻. A very dilute HCl might conduct comparably to a more concentrated salt solution.

Step 4: Rank Within the Weak Electrolyte Group.

• The degree of dissociation (α) is key. For weak acids, a lower pKₐ means stronger acid and greater dissociation. Acetic acid (pKₐ=4.76) is a stronger weak acid than phenol (pKₐ=9.95), so a 0.1 M acetic acid solution will have higher conductivity than a 0.1 M phenol solution.
• Concentration matters inversely. For weak electrolytes, dilution increases the percentage dissociation (per Ostwald's dilution law), but the absolute concentration of ions often peaks at a moderate concentration and decreases upon further dilution. A 0.01 M weak acid might have higher conductivity than a 0.001 M weak acid of the same type, despite the latter having a higher percentage of molecules dissociated.

Step 5: Rank Nonelectrolytes. All will have effectively zero conductivity and are tied for last place.

Example Ranking Problem

Rank the following 0.1 M aqueous solutions in order of increasing electrical conductivity:

1. HCl

2. CH₃COOH (acetic acid

3. NaCl

4. C₁₂H₂₂O₁₁ (sucrose)

5. CaCl₂

6. NH₃ (ammonia)

Solution:

Step 1 & 2: Classification

1. HCl: Strong acid → Strong electrolyte
2. CH₃COOH: Weak acid → Weak electrolyte
3. NaCl: Strong electrolyte
4. C₁₂H₂₂O₁₁: Nonelectrolyte
5. CaCl₂: Strong electrolyte
6. NH₃: Weak base → Weak electrolyte

Step 3: Strong Electrolyte Ranking

• NaCl: 0.1 M → 0.2 M total ions (1 Na⁺ + 1 Cl⁻)
• CaCl₂: 0.1 M → 0.3 M total ions (1 Ca²⁺ + 2 Cl⁻)
• HCl: 0.1 M → 0.2 M total ions (1 H⁺ + 1 Cl⁻), but H⁺ has exceptional mobility

Within strong electrolytes, CaCl₂ produces the most ions, so it ranks highest. HCl, despite having fewer total ions than CaCl₂, benefits from H⁺'s high mobility and will conduct better than NaCl.

Step 4: Weak Electrolyte Ranking

• CH₃COOH: Weak acid with moderate dissociation (pKₐ=4.76)
• NH₃: Weak base with moderate dissociation (pKₐ of NH₄⁺=9.25)

CH₃COOH is a stronger weak acid than NH₃ is a weak base, so CH₃COOH will have higher conductivity.

Step 5: Nonelectrolyte

• C₁₂H₂₂O₁₁: Zero conductivity

Final Ranking (Increasing Conductivity):

1. C₁₂H₂₂O₁₁ (sucrose) - Nonelectrolyte, zero conductivity
2. NH₃ (ammonia) - Weakest conductivity among weak electrolytes
3. CH₃COOH (acetic acid) - Stronger weak electrolyte than NH₃
4. NaCl - Strong electrolyte with moderate ion concentration
5. HCl - Strong acid with highly mobile H⁺ ions
6. CaCl₂ - Strong electrolyte with highest ion concentration (0.3 M total ions)

Conclusion

Understanding electrical conductivity in aqueous solutions is fundamental to chemistry and has broad practical applications. The ability of a solution to conduct electricity depends primarily on the presence and concentration of ions, with strong electrolytes providing the highest conductivity due to complete dissociation. Within strong electrolytes, the total ion concentration and the charge on those ions determine relative conductivity, while for weak electrolytes, the degree of dissociation becomes crucial. Nonelectrolytes, regardless of concentration, contribute nothing to conductivity.

This systematic approach—classifying by electrolyte strength, then ranking by ion concentration and mobility—provides a reliable framework for predicting and comparing the electrical behavior of solutions. Whether designing batteries, understanding biological systems, or analyzing water quality, these principles form the foundation for working with conductive solutions in both laboratory and real-world contexts.