Exercises: Hydrolysis of salts

Two flasks containing aqueous solutions of the same concentration are available in the laboratory: one of sodium chloride, $\mathrm{NaCl}$, and the other of potassium nitrite, $\mathrm{KNO_2}$, at $25\,^{\circ}\mathrm{C}$.

1. a) Write the equations for the dissociation of both salts in water and identify the spectator ions that do not undergo hydrolysis.
2. b) Justify the acidic, basic or neutral character of each of the two resulting solutions, given that $K_\mathrm{a}(\mathrm{HNO_2})=4.5\times 10^{-4}$ at $25\,^{\circ}\mathrm{C}$. Consider the solutions to be ideal.

Ammonium nitrate, $\mathrm{NH_4NO_3}$, is a compound widely used in fertiliser solutions at $25\,^{\circ}\mathrm{C}$.

1. a) Write the ionic dissociation and hydrolysis processes in water corresponding to this salt, indicating which species act as a Brønsted-Lowry acid or base.
2. b) Deduce, with justification, whether the pH of the resulting aqueous solution will be lower than, equal to or higher than $7$. Data at $25\,^{\circ}\mathrm{C}$: $K_\mathrm{b}(\mathrm{NH_3})=1.8\times 10^{-5}$. Consider the solutions to be ideal.

An aqueous solution of ammonium fluoride, $\mathrm{NH_4F}$, is prepared at $25\,^{\circ}\mathrm{C}$. Consider the following equilibrium constant data at that temperature: $K_\mathrm{a}(\mathrm{HF})=6.6\times 10^{-4}$, $K_\mathrm{b}(\mathrm{NH_3})=1.8\times 10^{-5}$ and $K_\mathrm{w}=1.0\times 10^{-14}$.

1. a) Write the ionic reactions that take place when both ions from the salt interact with water.
2. b) Compare quantitatively the constants of the hydrolysis equilibria of the ions present and determine whether the resulting solution is acidic, basic or neutral. Consider the solutions to be ideal.

In the general chemistry laboratory, four beakers numbered 1 to 4 have been labelled, each containing a $0.10\text{ M}$ aqueous solution at $25\,^{\circ}\mathrm{C}$ of the following substances: $\mathrm{NaCl}$, $\mathrm{CH_3COONa}$, $\mathrm{NH_4Cl}$ and $\mathrm{KNO_3}$. The diagram of the experimental set-up is shown below:

1. a) Classify each of the four salts, indicating whether the acid and the base from which it derives are strong or weak, and state which undergo hydrolysis.
2. b) Given that $K_\mathrm{a}(\mathrm{CH_3COOH})=1.8\times 10^{-5}$, $K_\mathrm{b}(\mathrm{NH_3})=1.8\times 10^{-5}$ and $K_\mathrm{w}=1.0\times 10^{-14}$ at $25\,^{\circ}\mathrm{C}$, arrange the four solutions in order of increasing pH, justifying your answer. Consider the solutions to be ideal.

An aqueous solution is prepared by dissolving $4.10\text{ g}$ of sodium acetate, $\mathrm{CH_3COONa}$, in distilled water to a total volume of $500\text{ mL}$ at $25\,^{\circ}\mathrm{C}$.

1. a) Write the equation for the chemical hydrolysis equilibrium and determine the value of the hydrolysis constant $K_\mathrm{h}$.
2. b) Calculate the molar concentration of all the species present at equilibrium ($\mathrm{Na^+}$, $\mathrm{CH_3COO^-}$, $\mathrm{CH_3COOH}$, $\mathrm{OH^-}$ and $\mathrm{H_3O^+}$) and determine the final pH of the solution.

Data at $25\,^{\circ}\mathrm{C}$: Atomic molar masses in $\text{g}\times\text{mol}^{-1}$: $\mathrm{C}=12.0$; $\mathrm{H}=1.0$; $\mathrm{O}=16.0$; $\mathrm{Na}=23.0$. $K_\mathrm{a}(\mathrm{CH_3COOH})=1.8\times 10^{-5}$; $K_\mathrm{w}=1.0\times 10^{-14}$. Consider the solutions to be ideal.

An aqueous solution of ammonium chloride, $\mathrm{NH_4Cl}$, of concentration $0.25\text{ M}$ is kept at $25\,^{\circ}\mathrm{C}$.

1. a) Set up the corresponding ionic hydrolysis equilibrium and calculate the degree of hydrolysis ($\alpha_\mathrm{h}$) of the ammonium ion in this solution.
2. b) Determine the concentration of $\mathrm{H_3O^+}$ ions at equilibrium and the resulting pH.

Data at $25\,^{\circ}\mathrm{C}$: $K_\mathrm{b}(\mathrm{NH_3})=1.8\times 10^{-5}$; $K_\mathrm{w}=1.0\times 10^{-14}$. Consider the solutions to be ideal.

$250\text{ mL}$ of an aqueous solution of sodium cyanide, $\mathrm{NaCN}$, is available, with an initial concentration of $0.40\text{ M}$ at $25\,^{\circ}\mathrm{C}$. This solution is then diluted by adding distilled water to a total volume of $2.50\text{ L}$.

1. a) Calculate the pH and the degree of hydrolysis ($\alpha_\mathrm{h1}$) of the cyanide anion in the original concentrated solution.
2. b) Calculate the pH and the new degree of hydrolysis ($\alpha_\mathrm{h2}$) in the solution after dilution, and justify, with reasoning, the effect that dilution has on the degree of hydrolysis, based on Le Chatelier's principle.

Data at $25\,^{\circ}\mathrm{C}$: $K_\mathrm{a}(\mathrm{HCN})=4.9\times 10^{-10}$; $K_\mathrm{w}=1.0\times 10^{-14}$. Consider the solutions to be ideal.

$0.10\text{ moles}$ of ammonium formate, $\mathrm{HCOONH_4}$, is dissolved in distilled water to a volume of $1.0\text{ L}$ at $25\,^{\circ}\mathrm{C}$. Both the ammonium cation and the formate anion undergo hydrolysis in aqueous solution.

1. a) Write the individual hydrolysis equilibria of both ions together with the overall equilibrium that is established, and deduce, from the expressions of the constants, the equation relating $[\mathrm{H_3O^+}]$ exclusively to $K_\mathrm{a}$, $K_\mathrm{b}$ and $K_\mathrm{w}$, assuming that the degrees of hydrolysis of the cation and the anion are practically equal.
2. b) Calculate the pH of the solution and determine the molar concentration of the species $\mathrm{NH_3}$ and $\mathrm{HCOOH}$ at equilibrium.

Data at $25\,^{\circ}\mathrm{C}$: $K_\mathrm{a}(\mathrm{HCOOH})=1.8\times 10^{-4}$; $K_\mathrm{b}(\mathrm{NH_3})=1.8\times 10^{-5}$; $K_\mathrm{w}=1.0\times 10^{-14}$. Consider the solutions to be ideal.

$50.0\text{ mL}$ of a solution of benzoic acid ($\mathrm{C_6H_5COOH}$) $0.0120\text{ M}$ is titrated with a solution of sodium hydroxide ($\mathrm{NaOH}$) $0.0080\text{ M}$ at $25\,^{\circ}\mathrm{C}$ until the stoichiometric equivalence point is reached exactly.

1. a) Calculate the volume of titrant needed to reach equivalence and the pH of the resulting solution at that point.
2. b) If distilled water is added to the solution obtained at the equivalence point until its total volume doubles, calculate the new pH and show quantitatively the change in the percentage of benzoate ion hydrolysed.

Data at $25\,^{\circ}\mathrm{C}$: $K_\mathrm{a}(\mathrm{C_6H_5COOH})=6.3\times 10^{-5}$; $K_\mathrm{w}=1.0\times 10^{-14}$. Consider the solutions to be ideal.