Conductivity of Electrolyte Solutions: Classification, Measurement, and Analysis

Conductivity of Electrolyte Solutions

Introduction to Electrolytes and Conductivity

Electrical conductivity in aqueous solutions is a key property used to classify substances as strong electrolytes, weak electrolytes, or non-electrolytes. This classification is based on the extent to which a substance dissociates into ions when dissolved in water, which directly affects its ability to conduct electricity.

• Strong Electrolytes: Substances that dissociate completely into ions in solution, resulting in high conductivity.

• Weak Electrolytes: Substances that partially dissociate into ions, resulting in moderate to low conductivity.

• Non-Electrolytes: Substances that do not dissociate into ions and thus do not conduct electricity.

Examples of each type include:

• Strong Electrolytes: Soluble ionic compounds (e.g., NaCl), strong acids (e.g., HNO3), and strong bases (e.g., NaOH).

• Weak Electrolytes: Weak acids (e.g., HF) and weak bases (e.g., NH3, NH4OH).

• Non-Electrolytes: Molecular compounds (e.g., CH4, CO2).

Dissociation Equations

The dissociation of electrolytes in water can be represented by chemical equations. The direction of the arrow indicates the extent of dissociation:

• Strong Electrolytes: Use a single arrow ($\rightarrow$) to show complete dissociation.

• Weak Electrolytes: Use a double arrow ($\rightleftharpoons$) to show partial and reversible dissociation.

Examples:

• NaCl(aq): $\mathrm{NaCl(aq) \rightarrow Na^+(aq) + Cl^-(aq)}$

• HNO3(aq): $\mathrm{HNO_3(aq) \rightarrow H^+(aq) + NO_3^-(aq)}$

• NaOH(aq): $\mathrm{NaOH(aq) \rightarrow Na^+(aq) + OH^-(aq)}$

• HF(aq): $\mathrm{HF(aq) \rightleftharpoons H^+(aq) + F^-(aq)}$

• NH4OH(aq): $\mathrm{NH_4OH(aq) \rightleftharpoons NH_4^+(aq) + OH^-(aq)}$

Additional info: For weak electrolytes, the majority of the substance remains undissociated in solution.

Measurement of Conductivity

Principles of Conductivity Measurement

Conductivity is measured in units of microSiemens per centimeter (µS/cm). The presence of ions (charged particles) in solution allows the flow of electric current, and the magnitude of conductivity is directly related to the concentration and mobility of these ions.

• Higher ion concentration leads to higher conductivity.

• Greater number of ions per formula unit (e.g., AlCl3 vs. NaCl) increases conductivity at the same molar concentration.

Experimental Procedure Overview

• Use a conductivity probe connected to a computer interface to measure the conductivity of various solutions.

• Rinse the probe with deionized water between measurements to avoid contamination.

• Classify each solution as a strong, weak, or non-electrolyte based on measured conductivity values.

• Investigate the effect of increasing molar concentration and the number of ions on conductivity using NaCl, CaCl2, and AlCl3 solutions.

Classification of Solutions by Conductivity

Typical Conductivity Results and Classification

Solution	Conductivity (µS/cm)	Classification
NaCl	(High)	Strong Electrolyte
CaCl2	(High)	Strong Electrolyte
AlCl3	(High)	Strong Electrolyte
HCl	(High)	Strong Electrolyte
H3PO4	(Moderate)	Weak Electrolyte
CH3COOH	(Low)	Weak Electrolyte
H3BO3	(Low)	Weak Electrolyte
CH3OH	(Very Low/Zero)	Non-Electrolyte
C2H6O2	(Very Low/Zero)	Non-Electrolyte
Tap Water	(Low)	Weak Electrolyte (due to dissolved ions)
Deionized Water	(Very Low/Zero)	Non-Electrolyte

Additional info: Actual conductivity values depend on concentration and purity of reagents.

Effect of Concentration and Number of Ions on Conductivity

Experimental Design and Data Collection

To study the effect of concentration and the number of ions, solutions of NaCl, CaCl2, and AlCl3 are prepared at the same molarity. Small, measured volumes are added to a fixed volume of deionized water, and conductivity is measured after each addition.

Trial	Volume Added (ml)	Concentration (M)	NaCl Conductivity (µS/cm)	CaCl2 Conductivity (µS/cm)	AlCl3 Conductivity (µS/cm)
1	0.0	0.00
2	0.1	0.00014
3	0.2	(Calculate as shown below)

Calculation of Final Concentration:

For each addition, use the dilution equation:

• $M_1V_1 = M_2V_2$

• Where $M_1$ = initial molarity, $V_1$ = volume added, $M_2$ = final molarity, $V_2$ = total volume after addition.

Example Calculation (Trial 2):

• $M_1 = 0.050\ \mathrm{M}$

• $V_1 = 0.100\ \mathrm{ml}$

• $V_2 = 35.1\ \mathrm{ml}$

• $0.050 \times 0.100 = M_2 \times 35.1$

• $M_2 = \frac{0.050 \times 0.100}{35.1} = 0.00014\ \mathrm{M}$

Graphical Analysis

Plot the conductivity (y-axis) versus the concentration of the electrolyte (x-axis) for each salt. The slope of each line reflects the relationship between ion concentration and conductivity.

• NaCl: Dissociates into 2 ions per formula unit ($\mathrm{Na^+}$ and $\mathrm{Cl^-}$).

• CaCl2: Dissociates into 3 ions per formula unit ($\mathrm{Ca^{2+}}$ and 2 $\mathrm{Cl^-}$).

• AlCl3: Dissociates into 4 ions per formula unit ($\mathrm{Al^{3+}}$ and 3 $\mathrm{Cl^-}$).

The greater the number of ions produced per formula unit, the steeper the slope of the conductivity vs. concentration line.

Dissociation Equations for Salts

• NaCl(aq): $\mathrm{NaCl(aq) \rightarrow Na^+(aq) + Cl^-(aq)}$

• CaCl2(aq): $\mathrm{CaCl_2(aq) \rightarrow Ca^{2+}(aq) + 2Cl^-(aq)}$

• AlCl3(aq): $\mathrm{AlCl_3(aq) \rightarrow Al^{3+}(aq) + 3Cl^-(aq)}$

Relationship Between Slope and Number of Ions

• The slope of the conductivity vs. concentration line increases with the number of ions produced per formula unit.

• AlCl3 (4 ions) > CaCl2 (3 ions) > NaCl (2 ions) in terms of slope magnitude.

Summary Table: Number of Ions and Conductivity

Electrolyte	Dissociation Equation	Number of Ions Produced	Relative Slope
NaCl	$\mathrm{NaCl(aq) \rightarrow Na^+(aq) + Cl^-(aq)}$	2	Lowest
CaCl2	$\mathrm{CaCl_2(aq) \rightarrow Ca^{2+}(aq) + 2Cl^-(aq)}$	3	Intermediate
AlCl3	$\mathrm{AlCl_3(aq) \rightarrow Al^{3+}(aq) + 3Cl^-(aq)}$	4	Highest

Key Conclusions

• Electrical conductivity in aqueous solutions is determined by the concentration of ions and the number of ions produced per formula unit.

• Strong electrolytes dissociate completely and conduct well; weak electrolytes dissociate partially and conduct poorly; non-electrolytes do not conduct.

• Increasing the molar concentration of an electrolyte increases conductivity.

• At the same molar concentration, electrolytes that produce more ions per formula unit (e.g., AlCl3) have higher conductivity than those that produce fewer ions (e.g., NaCl).

Example Application

If you are given two solutions of equal molarity, one of NaCl and one of AlCl3, the AlCl3 solution will have a higher conductivity because it produces more ions per formula unit upon dissociation.