Acids and bases
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Define the terms 'conjugate acid' and 'conjugate base'.
A conjugate acid is a species formed when a base accepts a proton (H⁺). A conjugate base is a species formed when an acid donates a proton (H⁺).
Identify the conjugate acid-base pairs in the following reaction: NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)
The conjugate acid-base pairs are: NH₃ (base) and NH₄⁺ (conjugate acid), and H₂O (acid) and OH⁻ (conjugate base).
Define pH mathematically and explain its significance.
pH is defined as -log₁₀[H⁺(aq)], where [H⁺(aq)] is the concentration of hydrogen ions in mol/L (or mol/dm³). pH indicates the acidity or alkalinity of a solution; lower pH values indicate higher acidity.
Define Ka and pKa mathematically. What does a smaller pKa value indicate?
Ka is the acid dissociation constant, measuring the strength of an acid. pKa = -log₁₀Ka. A smaller pKa value indicates a stronger acid, meaning it dissociates more readily in solution.
Define Kw mathematically and state its value at 298K.
Kw is the ionic product of water, defined as Kw = [H⁺(aq)][OH⁻(aq)]. At 298K (25°C), Kw = 1.0 x 10⁻¹⁴ mol² dm⁻⁶.
A solution of a strong acid, HCl, has a concentration of 0.01 mol/L. Calculate the pH of this solution.
Since HCl is a strong acid, it completely dissociates. Therefore, [H⁺(aq)] = 0.01 mol/L. pH = -log₁₀(0.01) = 2.
A solution of a strong alkali, NaOH, has a concentration of 0.005 mol/L. Calculate the pH of this solution at 298K.
First, calculate [OH⁻(aq)] = 0.005 mol/L. Then, calculate [H⁺(aq)] using Kw = [H⁺(aq)][OH⁻(aq)], so [H⁺(aq)] = Kw/[OH⁻(aq)] = (1.0 x 10⁻¹⁴) / 0.005 = 2 x 10⁻¹² mol/L. Finally, pH = -log₁₀(2 x 10⁻¹²) = 11.70.
Define the term 'buffer solution' and explain its function.
A buffer solution resists changes in pH upon the addition of small amounts of acid or base. It typically consists of a weak acid and its conjugate base (or a weak base and its conjugate acid).
Write the expression for the solubility product, Ksp, of silver chloride (AgCl).
The dissolution of silver chloride is represented as: AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq). The solubility product expression is: Ksp = [Ag⁺(aq)][Cl⁻(aq)].
Explain the common ion effect and how it affects solubility.
The common ion effect is the decrease in solubility of a sparingly soluble salt when a soluble salt containing a common ion is added to the solution. The added common ion shifts the equilibrium of the sparingly soluble salt towards precipitation, decreasing its solubility.
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