Question
\(H_2O(l)+H_2O(l)\rightleftharpoons H_3O^+(aq)+OH^-(aq)\) \( K_w=7.0\times 10^{−14}\) at 55°C
Pure water autoionizes as shown in the equation above. Based on this information, which of the following is correct?
A The autoionization equilibrium for pure water favors the formation of reactants more at 55°C than at 25°C.
B The autoionization equilibrium for pure water produces the same amount of OH− ions at 55°C and 25°C.
C At 55°C , \(pH =-log(\sqrt{K_w})\) for pure water.
D At 55°C , \(pH=−log(K_w)\) for pure water.
▶️Answer/Explanation
Ans:C
Regardless of the temperature, for pure water \(K_w=[H_3O^+][OH^-]\).
At 55°C, \([H_3O^+]=[OH^-]=\sqrt{7.0\times 10^{-14}}\) and \(pH =-log(\sqrt{K_w})\)
Question
\(K_w=[H_3O^+][OH^−]=1.0\times 10^{−14}\) at 25°C
Based on the information above, which of the following is true for a sample of pure water at 25°C ?
A \([H_3O^+]=7.0M\)
B \([OH^−]=1.0\times 10^{−14}M\)
C \(pH=10^{−7}\)
D \(pOH=7.00\)
▶️Answer/Explanation
Ans:D
\(pOH=−log[OH^−]\) , which is equal to \(−log (1.0\times 10^{−7})=7.00\) at 25°C.
Question
\(2H_2O(l)\rightleftharpoons H^3O^+(aq)+OH^-(aq) \) ΔH°\(=+56kJ/mol_{rxn}\)
The endothermic autoionization of pure water is represented by the chemical equation shown above. The pH of pure water is measured to be 7.00 at 25.0°C and 6.02 at 100.0°C. Which of the following statements best explains these observations?
A At the higher temperature water dissociates less, \([H_3O^+]<[OH^−]\), and the water becomes basic.
B At the higher temperature water dissociates less, \([H_3O^+]=[OH^−]\), and the water remains neutral.
C At the higher temperature water dissociates more, \([H_3O^+]>[OH^−]\), and the water becomes acidic.
D At the higher temperature water dissociates more, \([H_3O^+]=[OH^−]\), and the water remains neutral.
▶️Answer/Explanation
Ans:D
Because the reaction is endothermic, increasing temperature will increase \(K_w\). So the water dissociates more, and \([H_3O^+]\) and \([OH^−]\) will always be equal in pure water.
Question
\(HX(aq)+Y^{-} \rightleftharpoons X^{-}(aq) \) \(K_{eq}>1\)
Based on the information given above, which of the following is the strongest acid?
(A) HX(aq)
(B) \(Y^{-}\)(aq)
(C) HY(aq)
(D) \(X^{-}\)(aq)
▶️Answer/Explanation
Ans:C
Question
\(NaOH(aq)+HCl(aq) \rightarrow NaCl(aq)+H_{2}O(l)\)
To determine the concentration of a NaOH(aq) solution, a student titrated a 50. mL sample with 0.10 M HCl(aq). The reaction is represented by the equation above. The titration is monitored using a pH meter, and the experimental results are plotted in the graph below.
At the point labeled R on the pH curve, which of the following ions are present in the reaction mixture at a concentration greater than 0.01 M ?
(A)\( Na^+\) and \(Cl^{-} \)only
(B)\( Na^+\), \(Cl-^{-}\), and \(H^{+} \)only
(C) \(Na+, Cl^{-}\), and \(OH^{-} \)only
(D) \( Na^+, Cl^{-}\), \(H^+\), and OH^{-}\)
▶️Answer/Explanation
Ans:A