Question

Without doing any calculations, determine which solution in each pair is most basic. $$\begin{array}{l}{\text { a. } 0.100 \mathrm{M} \text { in } \mathrm{NaClO} \text { and } 0.100 \mathrm{M} \text { in } \mathrm{NaF}} \\ {\text { b. } 0.0100 \mathrm{M} \text { in } \mathrm{KCl} \text { and } 0.0100 \mathrm{M} \text { in } \mathrm{KClO}_{2}} \\ {\text { c. } 0.0100 \mathrm{M} \text { in } \mathrm{HNO}_{3} \text { and } 0.0100 \mathrm{M} \text { in NaOH }} \\\ {\text { d. } 0.0100 \mathrm{M} \text { in } \mathrm{NH}_{4} \mathrm{Cl} \text { and } 0.0100 \mathrm{M} \text { in HCN }}\end{array}$$

Short answer

The most basic solutions are: a) 0.100 M NaClO, b) 0.0100 M KClO2, c) 0.0100 M NaOH. Option d) does not contain a basic solution, but NH4Cl is less basic than HCN.

Step-by-step solution

Compare the Basicity of NaClO and NaF

NaClO is a salt formed from the weak acid hypochlorous acid (HClO) and the strong base NaOH, while NaF is formed from the weak acid hydrofluoric acid (HF) and the strong base NaOH. The anion from the weaker acid has a stronger tendency to accept a proton and thus acts as a better base. Since HF is a stronger acid than HClO, NaClO solution is more basic than NaF.

Compare the Basicity of KCl and KClO2

KCl is a salt of a strong acid (HCl) and a strong base (KOH), and thus is neutral. KClO2 is a salt formed from a weak acid chlorous acid (HClO2) and a strong base KOH. Since KCl is neutral, a solution of KClO2, which can generate basic anions, will be more basic.

Compare the Basicity of HNO3 and NaOH

HNO3 is a strong acid, and its solution is highly acidic. NaOH is a strong base, and its solution is highly basic. Since we are looking for the more basic solution, NaOH solution is more basic than HNO3.

Compare the Basicity of NH4Cl and HCN

NH4Cl is a salt formed from the weak base ammonia (NH3) and a strong acid HCl, making it slightly acidic. HCN is a weak acid. As a result, an NH4Cl solution is less basic than pure water, while the presence of HCN indicates acidity. Neither solution is basic, but NH4Cl is less basic compared to HCN, which is acidic and thus has no basicity.

Key concepts

Weak and Strong Acids

Understanding the difference between weak and strong acids is essential in predicting the behavior of solutions. A strong acid is one that completely dissociates in water, releasing all its protons (H⁺) into the solution. Examples include hydrochloric acid (HCl) and nitric acid (HNO₃). On the other hand, a weak acid only partially dissociates in water, which means only some of its protons are released. Acids like acetic acid (CH₃COOH) and hydrofluoric acid (HF) are weak acids. The basicity of a solution can be influenced by the presence of weak or strong acids. In the given exercise, for instance, HNO₃ is a strong acid and thus makes solution c highly acidic, much less basic than a solution of NaOH, which is a strong base.

In summary, strong acids create more H⁺ ions in solution, leading to lower pH values, while weak acids contribute to higher pH levels due to their incomplete dissociation.

Weak and Strong Bases

Like acids, bases are categorized as weak or strong based on their ability to dissociate in water. A strong base, such as sodium hydroxide (NaOH) or potassium hydroxide (KOH), dissociates completely, liberating OH^- ions into the solution. A weak base, such as ammonia (NH₃), doesn't fully dissociate, providing fewer OH^- ions.

This difference is vital when determining a solution's basicity. For instance, solution a involves NaClO, formed from a reaction involving a weak acid (HClO) and a strong base (NaOH). Given that strong bases produce more OH^- ions, it contributes to a more basic solution compared to NaF, which results from a stronger acid (HF) and the same strong base (NaOH). The presence of a strong base in a salt's formation generally indicates greater basicity.

Acid-Base Neutralization

The concept of acid-base neutralization is fundamental to understanding solution chemistry. When an acid and a base react, they form water and a salt in a neutralization reaction. The pH of the resulting solution depends on the strengths of the acid and base involved. If a strong acid reacts with a strong base, the solution is typically neutral. However, when a weak acid reacts with a strong base, or a weak base reacts with a strong acid, the resulting solution may be acidic or basic, respectively.

For example, in solution d, NH₄Cl is formed from the reaction of a weak base (ammonia) with a strong acid (HCl), leading to a slightly acidic solution. In contrast, KClO₂ in solution b results from the reaction of a weak acid (HClO₂) with a strong base (KOH), which creates a more basic solution. These neutralization reactions directly influence the basicity or acidity of the compounds formed in solution.

Solution pH

The pH is a measure of the acidity or basicity of a solution. It is a logarithmic scale that expresses the concentration of hydrogen ions (H⁺) in a solution. The pH scale ranges from 0 to 14, with 7 being neutral. Values below 7 indicate acidity, and values above 7 indicate basicity. A strong acid solution has a low pH, close to 0, while a strong base solution has a high pH, close to 14.

In the context of the exercise, we expect NaOH (solution c) to have a high pH due to its strong basic nature, and HNO₃ to have a low pH because of its strong acidic nature. Understanding how the pH scale works allows us to predict the relative acidity or basicity of the given solutions without calculations, as the strength of acids and bases directly influences the pH of their respective solutions.