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
On a mountaintop, it is observed that water boils at 90°C not at 100°C as at sea level. This phenomenon occurs because on the mountaintop the
(A) equilibrium water vapor pressure is higher due to the higher atmospheric pressure
(B) equilibrium water vapor pressure is lower due to the higher atmospheric pressure
(C) equilibrium water vapor pressure equals the atmospheric pressure at a lower temperature
(D) water molecules have a higher average kinetic energy due to the lower atmospheric pressure
(E) water contains a greater concentration of dissolved gases
▶️Answer/Explanation
Ans: C