Questions: Acids and Bases Predicting the qualitative acid-base properties of salts Consider the following data on some weak acids and weak bases: - acid - name: hydrocyanic acid, formula: HCN, Ka: 4.9 x 10^-10 - name: nitrous acid, formula: HNO2, Ka: 4.5 x 10^-4 - base - name: methylamine, formula: CH3NH2, Kb: 4.4 x 10^-4 - name: hydroxylamine, formula: HONH2, Kb: 1.1 x 10^-8 Use this data to rank the following solutions in order of increasing pH. In other words, select a '1' next to the solution that will have the lowest pH, a '2' next to the solution that will have the next lowest pH, and so on. - 0.1 M CH3NH3Cl: choose one - 0.1 M KCN: choose one - 0.1 M KI: choose one - 0.1 M NaNO2: choose one

Acids and Bases Predicting the qualitative acid-base properties of salts Consider the following data on some weak acids and weak bases: - acid - name: hydrocyanic acid, formula: HCN, Ka: 4.9 x 10^-10 - name: nitrous acid, formula: HNO2, Ka: 4.5 x 10^-4 - base - name: methylamine, formula: CH3NH2, Kb: 4.4 x 10^-4 - name: hydroxylamine, formula: HONH2, Kb: 1.1 x 10^-8 Use this data to rank the following solutions in order of increasing pH. In other words, select a '1' next to the solution that will have the lowest pH, a '2' next to the solution that will have the next lowest pH, and so on. - 0.1 M CH3NH3Cl: choose one - 0.1 M KCN: choose one - 0.1 M KI: choose one - 0.1 M NaNO2: choose one
Transcript text: Acids and Bases Predicting the qualitative acid-base properties of salts Consider the following data on some weak acids and weak bases: \begin{tabular}{|c|c|c|} \hline \multicolumn{2}{|c|}{ acid } & \multirow{2}{*}{$K_{a}$} \\ \hline name & formula & \\ \hline hydrocyanic acid & HCN & $4.9 \times 10^{-10}$ \\ \hline nitrous acid & $\mathrm{HNO}_{2}$ & $4.5 \times 10^{-4}$ \\ \hline \end{tabular} \begin{tabular}{|c|c|c|} \hline \multicolumn{2}{|c|}{ base } & \multirow{2}{*}{$K_{b}$} \\ \hline name & formula & \\ \hline methylamine & $\mathrm{CH}_{3} \mathrm{NH}_{2}$ & $4.4 \times 10^{-4}$ \\ \hline hydroxylamine & $\mathrm{HONH}_{2}$ & $1.1 \times 10^{-8}$ \\ \hline \end{tabular} Use this data to rank the following solutions in order of increasing pH . In other words, select a ' 1 ' next to the solution that will have the lowest pH , a ' 2 ' next to the solution that will have the next lowest pH, and so on. \begin{tabular}{|l|l|} \hline \multicolumn{1}{|c|}{ solution } & \multicolumn{1}{c|}{pH} \\ \hline $0.1 \mathrm{M} \mathrm{CH}_{3} \mathrm{NH}_{3} \mathrm{Cl}$ & choose one $\vee$ \\ \hline 0.1 M KCN & choose one $\checkmark$ \\ \hline 0.1 M KI & choose one $\checkmark$ \\ \hline $0.1 \mathrm{M} \mathrm{NaNO}_{2}$ & choose one $\checkmark$ \\ \hline \end{tabular}
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Solution

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Solution Steps

Step 1: Identify the nature of each salt
  • \( \mathrm{CH}_3\mathrm{NH}_3\mathrm{Cl} \): This is a salt of a weak base (\(\mathrm{CH}_3\mathrm{NH}_2\)) and a strong acid (HCl). It will produce an acidic solution.
  • \( \mathrm{KCN} \): This is a salt of a weak acid (HCN) and a strong base (KOH). It will produce a basic solution.
  • \( \mathrm{KI} \): This is a salt of a strong acid (HI) and a strong base (KOH). It will produce a neutral solution.
  • \( \mathrm{NaNO}_2 \): This is a salt of a weak acid (\(\mathrm{HNO}_2\)) and a strong base (NaOH). It will produce a basic solution.
Step 2: Determine the relative pH of each solution
  • \( \mathrm{CH}_3\mathrm{NH}_3\mathrm{Cl} \): Since it is derived from a weak base and a strong acid, it will have the lowest pH (most acidic).
  • \( \mathrm{KCN} \): Since it is derived from a weak acid and a strong base, it will have a higher pH (basic).
  • \( \mathrm{KI} \): Since it is derived from a strong acid and a strong base, it will be neutral (pH = 7).
  • \( \mathrm{NaNO}_2 \): Since it is derived from a weak acid and a strong base, it will have a higher pH (basic).
Step 3: Rank the solutions in order of increasing pH
  • \( \mathrm{CH}_3\mathrm{NH}_3\mathrm{Cl} \): Most acidic, so it will have the lowest pH.
  • \( \mathrm{KI} \): Neutral, so it will have a pH of 7.
  • \( \mathrm{KCN} \) and \( \mathrm{NaNO}_2 \): Both are basic, but we need to compare their \( K_b \) values to determine which is more basic.
Step 4: Compare the basicity of \( \mathrm{KCN} \) and \( \mathrm{NaNO}_2 \)
  • For \( \mathrm{KCN} \), the conjugate base is \( \mathrm{CN}^- \) with \( K_b = \frac{K_w}{K_a} = \frac{1.0 \times 10^{-14}}{4.9 \times 10^{-10}} = 2.04 \times 10^{-5} \).
  • For \( \mathrm{NaNO}_2 \), the conjugate base is \( \mathrm{NO}_2^- \) with \( K_b = \frac{K_w}{K_a} = \frac{1.0 \times 10^{-14}}{4.5 \times 10^{-4}} = 2.22 \times 10^{-11} \).

Since \( \mathrm{KCN} \) has a higher \( K_b \) value than \( \mathrm{NaNO}_2 \), \( \mathrm{KCN} \) will be more basic.

Final Answer

\[ \begin{array}{|l|l|} \hline \text{solution} & \text{pH} \\ \hline 0.1 \mathrm{M} \mathrm{CH}_{3} \mathrm{NH}_{3} \mathrm{Cl} & \boxed{1} \\ \hline 0.1 \mathrm{M} \mathrm{KI} & \boxed{2} \\ \hline 0.1 \mathrm{M} \mathrm{NaNO}_{2} & \boxed{3} \\ \hline 0.1 \mathrm{M} \mathrm{KCN} & \boxed{4} \\ \hline \end{array} \]

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