Question

Arrange the following \(0.10 \mathrm{M}\) solutions in order of increasing acidity (decreasing pH): (i) \(\mathrm{NH}_{4} \mathrm{NO}_{3}\), (ii) \(\mathrm{NaNO}_{3}\), (iii) \(\mathrm{CH}_{3} \mathrm{COONH}_{4}\) (iv) \(\mathrm{NaF}\), (v) \(\mathrm{CH}_{3} \mathrm{COONa}\)

Short answer

The solutions arranged in order of increasing acidity (decreasing pH) are: (iv) \(\mathrm{NaF}\), (v) \(\mathrm{CH}_{3} \mathrm{COONa}\), (ii) \(\mathrm{NaNO}_{3}\), (iii) \(\mathrm{CH}_{3} \mathrm{COONH}_{4}\), and (i) \(\mathrm{NH}_{4} \mathrm{NO}_{3}\).

Step-by-step solution

Identify Acidic, Basic or Neutral Compounds in the List

(i) \(\mathrm{NH}_{4} \mathrm{NO}_{3}\): This compound dissociates into \(\mathrm{NH}_{4}^{+}\) and \(\mathrm{NO}_{3}^{-}\) ions. \(\mathrm{NH}_{4}^{+}\) is the conjugate acid of the weak base \(\mathrm{NH}_{3}\) and will make the solution acidic. The ion \(\mathrm{NO}_{3}^{-}\) does not hydrolyze in water, so it does not contribute to the acidity or basicity of the solution. (ii) \(\mathrm{NaNO}_{3}\): This compound dissociates into \(\mathrm{Na}^{+}\) and \(\mathrm{NO}_{3}^{-}\) ions. The ions \(\mathrm{Na}^{+}\) and \(\mathrm{NO}_{3}^{-}\) do not hydrolyze in water, so they do not contribute to the acidity or basicity of the solution, making it a neutral solution. (iii) \(\mathrm{CH}_{3} \mathrm{COONH}_{4}\): This compound dissociates into \(\mathrm{NH}_{4}^{+}\) and \(\mathrm{CH}_{3}\mathrm{COO}^{-}\) ions. The ion \(\mathrm{NH}_{4}^{+}\) is the conjugate acid of the weak base \(\mathrm{NH}_{3}\) and will make the solution acidic. The ion \(\mathrm{CH}_{3}\mathrm{COO}^{-}\) is the conjugate base of the weak acid \(\mathrm{CH}_{3}\mathrm{COOH}\), but its effect on the acidity/basicity of the solution will be less than that of \(\mathrm{NH}_{4}^{+}\). (iv) \(\mathrm{NaF}\): This compound dissociates into \(\mathrm{Na}^{+}\) and \(\mathrm{F}^{-}\) ions. The ion \(\mathrm{Na}^{+}\) does not hydrolyze in water, so it does not contribute to the acidity or basicity of the solution. The ion \(\mathrm{F}^{-}\) is the conjugate base of the weak acid \(\mathrm{HF}\) and will make the solution basic. (v) \(\mathrm{CH}_{3} \mathrm{COONa}\): This compound dissociates into \(\mathrm{Na}^{+}\) and \(\mathrm{CH}_{3}\mathrm{COO}^{-}\) ions. The ion \(\mathrm{CH}_{3}\mathrm{COO}^{-}\) is the conjugate base of the weak acid \(\mathrm{CH}_{3}\mathrm{COOH}\) and will make the solution basic. The ion \(\mathrm{Na}^{+}\) does not hydrolyze in water, so it does not contribute to the acidity or basicity of the solution.

Arrange the Solutions in Order of Increasing Acidity

Now that we have identified the acidic, basic and neutral nature of the given solutions, we can arrange them in order of increasing acidity (decreasing pH values). Based on the analysis, we can arrange the solutions in the following order: 1. Most acidic (lowest pH): (i) \(\mathrm{NH}_{4} \mathrm{NO}_{3}\) 2. More acidic: (iii) \(\mathrm{CH}_{3} \mathrm{COONH}_{4}\) 3. Neutral: (ii) \(\mathrm{NaNO}_{3}\) 4. More basic: (v) \(\mathrm{CH}_{3} \mathrm{COONa}\) 5. Most basic (highest pH): (iv) \(\mathrm{NaF}\) Thus, the solutions, arranged in order of increasing acidity, are: (iv) \(\mathrm{NaF}\), (v) \(\mathrm{CH}_{3} \mathrm{COONa}\), (ii) \(\mathrm{NaNO}_{3}\), (iii) \(\mathrm{CH}_{3} \mathrm{COONH}_{4}\), and (i) \(\mathrm{NH}_{4} \mathrm{NO}_{3}\).

Key concepts

Acidity

Acidity refers to the presence and concentration of hydrogen ions \( \left( \ce{H^+} \right) \) in a solution. A higher concentration of hydrogen ions results in a lower pH value, indicating a more acidic solution. When a compound dissociates in water, it releases ions that might increase the concentration of \( \ce{H^+} \). For example, ammonium nitrate \( \ce{NH4NO3} \) dissociates into \( \ce{NH4+} \) and \( \ce{NO3-} \) ions. The ammonium ion \( \ce{NH4+} \) acts as a weak acid because it can donate a proton to water molecules, increasing the solution's acidity. Therefore, when ranking substances by acidity, we consider how their ion dissociation contributes to the hydrogen ion concentration.

Basicity

Basicity measures the ability of a solution to capture hydrogen ions, often described as the concentration of hydroxide ions \( \ce{OH^-} \). Basic solutions have higher pH values due to lower concentrations of \( \ce{H^+} \) ions. Compounds like sodium fluoride \( \ce{NaF} \) dissociate into sodium ions \( \ce{Na^+} \) and fluoride ions \( \ce{F^-} \). The fluoride ion can accept a hydrogen ion from water, forming the weak acid hydrofluoric acid \( \ce{HF} \), thus leaving behind excess hydroxide ions \( \ce{OH^-} \). This process makes the solution basic.To judge basicity, we examine ions that absorb \( \ce{H^+} \), impacting the available \( \ce{OH^-} \) ions in the solution.

Neutral Solutions

Neutral solutions have equal concentrations of hydrogen and hydroxide ions. This balance results in a pH of 7, indicating a neutral state. The dissociation of compounds such as sodium nitrate \( \ce{NaNO3} \) results in sodium ions \( \ce{Na^+} \) and nitrate ions \( \ce{NO3^-} \), neither of which significantly interacts with water to alter the ion balance. Thus, \( \ce{NaNO3} \) solutions remain neutral because these ions do not partake in reactions that either increase \( \ce{H^+} \) or \( \ce{OH^-} \) concentrations.When deciding if a solution is neutral, we need to check if the dissolved species interact with water to modify acidity or basicity.

Ion Dissociation

Ion dissociation involves the separation of a compound into its individual ions in solution. This process is fundamental in determining a solution's pH, as ions can affect the \( \ce{H^+} \) or \( \ce{OH^-} \) concentration. Take sodium acetate \( \ce{CH3COONa} \) for instance, it dissociates into sodium ions \( \ce{Na^+} \) and acetate ions \( \ce{CH3COO^-} \). The acetate ion reacts with water, slightly increasing the solution's pH by decreasing \( \ce{H^+} \) ions.To fully grasp how solutions behave, it's crucial to understand how different ions from a compound will interact with water. This interaction underlies whether a solution will be acidic, basic, or neutral.