Multiple Choice
When two water molecules undergo auto-ionization (self-ionization), what are the products formed?
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17. Acid and Base Equilibrium
Auto-Ionization
Multiple Choice
Pure water undergoes auto-ionization according to . Why does pure water have a neutral pH at ?
A
Because pure water contains no ions at all, so its pH must be neutral.
B
Because the auto-ionization equilibrium always makes in pure water.
C
Because auto-ionization produces equal concentrations of and , so .
D
Because is an acid that fully dissociates, making the pH exactly .
Verified step by step guidance1
Recognize that pure water undergoes auto-ionization according to the equilibrium: \[2\ H_2O(l) \rightleftharpoons H_3O^+(aq) + OH^-(aq)\] This means water molecules react with each other to form hydronium and hydroxide ions.
Understand that at 25°C, the concentrations of hydronium ions \([H_3O^+]\) and hydroxide ions \([OH^-]\) produced by auto-ionization are equal because the reaction produces them in a 1:1 ratio.
Recall that pH is defined as \[pH = -\log[H_3O^+]\] and pOH is defined as \[pOH = -\log[OH^-]\], and that for pure water at 25°C, \([H_3O^+] = [OH^-]\).
Use the relationship between pH and pOH at 25°C: \[pH + pOH = 14\]. Since \([H_3O^+] = [OH^-]\), it follows that \[pH = pOH = 7\], which is considered neutral.
Conclude that pure water has a neutral pH at 25°C because the auto-ionization equilibrium produces equal concentrations of \(H_3O^+\) and \(OH^-\) ions, balancing the solution and resulting in a pH of 7.
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