Question
A student measures the pH of a 0.0100M buffer solution made with \(HClO\) and \(NaClO\), as shown above. The \(pK_a\) of \(HClO\) is 7.40 at 25°C. Based on this information, which of the following best compares the relative concentrations of \(ClO^−\) and \(HClO\) in the buffer solution?
A \([ClO^−]=[HClO]\)
B \( [ClO^−]>[HClO]\)
C \([ClO^−]<[HClO]\)
D It is not possible to compare the concentrations without knowing the \(pK_b\) of \(ClO^−\).
▶️Answer/Explanation
Ans:B
\( [ClO^−]>[HClO]\). When the \(pH>pK_a\), the base form has a higher concentration than the acid form.
Question
\(HCNO\) is a weak acid with a \(pK_a\) value of 3.5. The graph above shows the results of a titration of an aqueous solution of \(HCNO\) with 0.100M \(NaOH\). Based on the results, the concentration of \(CNO^−\) is greater than the concentration of \(HCNO\) at which of the following pH values?
A 2.0
B 3.0
C 3.5
D 4.0
▶️Answer/Explanation
Ans:D
When the \(pH>pK_a\), the base form has a higher concentration than the acid form. This is true at pH 4.0, which is greater than the \(pK_a\) value of 3.5.
Question
▶️Answer/Explanation
Ans:B
When \(pH<pK_a\) , the acid form, \(C_2H_3COOH\), has a higher concentration than the base form, \(C_2H_3COO^−\). This is true because 3.55<4.25.
Question
Data collected during the titration of a 20.0 mL sample of a 0.10 M solution of a monoprotic acid with a solution of NaOH of unknown concentration are plotted in the graph above. Based on the data, which of the following are the approximate pK of the acid and the molar concentration of the NaOH?
\(pK_{a}\) [NaOH]
(A) 4.7 0.050 M
(B) 4.7 0.10 M
(C) 9.3 0.050 M
(D) 9.3 0.10 M
▶️Answer/Explanation
Ans:B
Question
\(HN0_{2}(aq)\rightleftharpoons H^{+}N0_{2}^{-}(aq)\) \(K_{a}=4.0\times 10^{-4}\)
On the basis of the information above, what is the approximate percent ionization of \(HNO_{2}\) in a 1.0 M\( HNO_{2}\)(aq) solution?
(A) 0.00040%
(B) 0.020%
(C) 0.040%
(D) 2.0%
▶️Answer/Explanation
Ans:C