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Autoionization of water Knowledge-Based Practice

Chemistry · Acids & Bases · Chemistry · Acid–Base Concepts

5 Chemistry practice questions on Autoionization of water. Questions that assess core concepts and understanding.

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Questions

  1. 1.

    When a small amount of a strong acid is dissolved in pure water at constant temperature, how does the autoionization equilibrium of water respond?

    • A) It shifts in the forward direction, increasing the concentration of dissolved hydroxide ions.
    • B) It maintains equal concentrations of hydronium and hydroxide ions by altering the value of KwK_w.
    • C) It shifts in the reverse direction, decreasing the concentration of hydroxide ions so that the ion product remains constant.
    • D) It remains completely unaffected because water is the solvent and its concentration is constant.
  2. 2.

    Why is the concentration of liquid water omitted from the denominator of the equilibrium constant expression for the autoionization of water, resulting in the expression Kw=[H3O+][OH−]K_w = [\text{H}_3\text{O}^+][\text{OH}^-]?

    • A) Water molecules act only as an inert solvent and do not chemically participate in the equilibrium process.
    • B) Water behaves as a strong electrolyte that dissociates almost completely into ions in dilute solution.
    • C) Liquid water has an unpredictable concentration that changes drastically as autoionization proceeds.
    • D) The concentration of water is overwhelmingly large and remains essentially constant during ionization.
  3. 3.

    In the autoionization of water, represented by 2H2O(l)⇌H3O+(aq)+OH−(aq)2\text{H}_2\text{O}(l) \rightleftharpoons \text{H}_3\text{O}^+(aq) + \text{OH}^-(aq), which statement correctly describes the Brønsted–Lowry behavior of the reacting water molecules?

    • A) Both reacting water molecules act as Brønsted–Lowry acids by donating protons.
    • B) Both reacting water molecules act as Brønsted–Lowry bases by accepting protons.
    • C) One water molecule acts as a Brønsted–Lowry acid by donating a proton, while the other acts as a Brønsted–Lowry base by accepting that proton.
    • D) Neither water molecule acts as an acid or a base because autoionization is a non-acid–base dissociation process.
  4. 4.

    At 25 °C, an aqueous solution has a hydronium ion concentration of 2.0×10−5 M2.0 \times 10^{-5}\text{ M}. What is the concentration of hydroxide ions in this solution?

    • A) 2.0×10−9 M2.0 \times 10^{-9}\text{ M}
    • B) 1.0×10−7 M1.0 \times 10^{-7}\text{ M}
    • C) 5.0×10−10 M5.0 \times 10^{-10}\text{ M}
    • D) 5.0×10−9 M5.0 \times 10^{-9}\text{ M}
  5. 5.

    Because the autoionization of water is an endothermic process, how do the value of KwK_w and the pH of pure water change as temperature increases?

    • A) KwK_w decreases and the pH increases, making the water slightly basic.
    • B) KwK_w increases and the pH increases, keeping the water neutral.
    • C) KwK_w increases and the pH decreases, keeping the water neutral.
    • D) KwK_w decreases and the pH decreases, making the water slightly acidic.

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