Question

Which of the following aqueous solutions would you expect to be the best conductor of electricity at \(25^{\circ} \mathrm{C} ?\) Explain your answer. a) \(0.20 \mathrm{M} \mathrm{NaCl}\) b) \(0.60 \mathrm{M} \mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\) c) \(0.25 \mathrm{M} \mathrm{HCl}\) d) \(0.20 \mathrm{M} \mathrm{Mg}\left(\mathrm{NO}_{3}\right)_{2}\)

Short answer

0.20 M Mg(NO₃)₂ is the best conductor due to the highest ion concentration.

Step-by-step solution

Assess Ionic Nature and Concentration

Conductivity in solutions is primarily determined by the presence of ions. The more ions a solution contains, the better it conducts electricity. First, identify the electrolyte type and concentration for each solution to determine the ion concentration.

Determine Ionization and Total Ion Count

- **NaCl** dissociates completely in water forming two ions, Na⁺ and Cl⁻. So, a 0.20 M NaCl solution has 0.40 M total ions. - **HC₂H₃O₂** (acetic acid) is a weak acid and only partially ionizes in water. Thus, it provides fewer ions than a strong acid or salt. - **HCl** is a strong acid; it completely ionizes in water to form H⁺ and Cl⁻ ions. A 0.25 M HCl solution produces 0.50 M total ions. - **Mg(NO₃)₂** dissociates completely into three ions: Mg²⁺ and 2NO₃⁻. A 0.20 M solution of Mg(NO₃)₂ results in 0.60 M total ions.

Compare Total Ion Concentrations

Compare the total ion concentrations: - NaCl: 0.40 M - HC₂H₃O₂: less than 0.60 M (due to partial ionization) - HCl: 0.50 M - Mg(NO₃)₂: 0.60 M The solution with the highest ion concentration will be the best conductor.

Identify the Best Conductor

The solution with the highest ion concentration is Mg(NO₃)₂ with 0.60 M ions. Thus, a 0.20 M Mg(NO₃)₂ solution is the best conductor of electricity among the given options, as it provides the most ions for electrical conductivity.

Key concepts

Ionic Dissociation

When compounds dissolve in water, they can break apart into ions through a process called ionic dissociation.
This is crucial for understanding how ionic compounds conduct electricity because only free-moving ions can carry an electric current in a solution.
In strong electrolytes like sodium chloride (NaCl), dissociation is complete, meaning the compound separates entirely into ions (Na⁺ and Cl⁻). Weak electrolytes, such as acetic acid (HC₂H₃O₂), only partially dissociate in water, which means not all of the molecules break into ions.
This results in fewer ions floating freely, thus offering lesser conductivity compared to fully dissociated strong electrolytes like hydrochloric acid (HCl). Understanding the level of dissociation helps predict the electrical conductivity of a solution. A solution with a higher degree of ionic dissociation will allow more ions to move freely and conduct more electricity efficiently.

Solution Concentration

Solution concentration refers to the amount of solute dissolved in a given volume of solvent, often expressed in molarity (M).
More concentrated solutions contain more solute particles, which in the context of ionic solutions, translates to more ions available for conducting electricity.
Molarity is calculated by dividing the moles of solute by the liters of solution, giving a direct way to quantify the solution's makeup. For instance, a 0.60 M solution of Mg(NO₃)₂ contains more ions overall than a 0.20 M NaCl solution because Mg(NO₃)₂ dissociates into three ions per formula unit (Mg²⁺ and 2NO₃⁻).
This results in a higher total ion concentration, making the Mg(NO₃)₂ solution a better conductor despite the lower concentration of solute compared to other substances such as HCl, which dissociates into two ions (H⁺ and Cl⁻).

Ionization Strength

Ionization strength describes how well a substance reacts with a solvent, like water, to form ions.
Strong electrolytes, such as HCl, ionize completely in solution, generating more ions and resulting in higher conductivity.
In contrast, weak electrolytes like CH₃COOH (acetic acid) only ionize partially, meaning they produce fewer ions and therefore less electrical conductivity. The difference in ionization strength between substances impacts their ability to conduct electricity. Strong ions readily break into their ionic components, ensuring many charged particles are available to carry an electric current.
Weak electrolytes, due to their partial ionization, have a lower concentration of ions and thus lower conductivity. These characteristics help distinguish between varying ionic behaviors in solution.

Aqueous Solutions

Aqueous solutions are solutions where water serves as the solvent.
In such solutions, the behavior of ions can be dramatically different compared to their behavior in a solid state.
Water is an exceptional solvent for ionic compounds because of its polar nature, allowing it to stabilize ions that are formed by dissociation. When compounds like NaCl are placed in water, they disassociate completely into their respective ions.
This change from a solid lattice to free-moving ions significantly augments their conductive capability. Excessive ion separation is what allows currents to pass through aqueous solutions effectively.
Therefore, the role of water in dissociating ions into an agile and conductive state is a crucial aspect in understanding electric conductivity.