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Acids and Bases - Strong and Weak Acids and Bases

Grade 9IB

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The strength of an acid or base is determined by its degree of ionization or dissociation in aqueous solution. It is not the same as concentration (molarity).

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A Strong Acid is one that undergoes complete dissociation in water. For example, in a solution of Hydrochloric acid, every molecule of HClHCl breaks into ions: HCl(aq)→H+(aq)+Cl−(aq)HCl(aq) \rightarrow H^+(aq) + Cl^-(aq).

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A Weak Acid only partially dissociates in water, establishing an equilibrium between the molecules and the ions. For example, Ethanoic acid: CH3COOH(aq)⇌CH3COO−(aq)+H+(aq)CH_3COOH(aq) \rightleftharpoons CH_3COO^-(aq) + H^+(aq).

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A Strong Base fully dissociates into its constituent ions in solution, such as Sodium Hydroxide: NaOH(aq)→Na+(aq)+OH−(aq)NaOH(aq) \rightarrow Na^+(aq) + OH^-(aq).

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A Weak Base partially reacts with water or dissociates to produce hydroxide ions, such as Ammonia: NH3(aq)+H2O(l)⇌NH4+(aq)+OH−(aq)NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq).

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Electrical conductivity is higher in strong acids and bases because they produce a higher concentration of mobile ions compared to weak acids and bases of the same concentration.

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The pHpH scale measures the concentration of hydrogen ions [H+][H^+]. Stronger acids have a higher [H+][H^+] and thus a lower pHpH value for the same concentration.

📐Formulae

pH=−log⁡10[H+]pH = -\log_{10}[H^+]

[H+]=10−pH[H^+] = 10^{-pH}

HA(aq)→H+(aq)+A−(aq) (Strong Acid Dissociation)HA(aq) \rightarrow H^+(aq) + A^-(aq) \text{ (Strong Acid Dissociation)}

HA(aq)⇌H+(aq)+A−(aq) (Weak Acid Dissociation)HA(aq) \rightleftharpoons H^+(aq) + A^-(aq) \text{ (Weak Acid Dissociation)}

BOH(aq)→B+(aq)+OH−(aq) (Strong Base Dissociation)BOH(aq) \rightarrow B^+(aq) + OH^-(aq) \text{ (Strong Base Dissociation)}

💡Examples

Problem 1:

Calculate the hydrogen ion concentration [H+][H^+] of a solution with pH=3pH = 3 and compare it to a solution with pH=5pH = 5.

Solution:

For pH=3pH = 3: [H+]=10−3 mol/dm3=0.001 mol/dm3[H^+] = 10^{-3} \text{ mol/dm}^3 = 0.001 \text{ mol/dm}^3 For pH=5pH = 5: [H+]=10−5 mol/dm3=0.00001 mol/dm3[H^+] = 10^{-5} \text{ mol/dm}^3 = 0.00001 \text{ mol/dm}^3 Difference in concentration: 0.00100−0.000010.00099\begin{array}{r} 0.00100 \\ -0.00001 \\ \hline 0.00099 \end{array} The solution with pH=3pH = 3 has 102=10010^2 = 100 times more H+H^+ ions than the solution with pH=5pH = 5.

Explanation:

The pH scale is logarithmic. Each decrease of 11 unit on the pH scale represents a tenfold increase in H+H^+ ion concentration.

Problem 2:

Why does 0.1 mol/dm30.1 \text{ mol/dm}^3 HClHCl have a lower pHpH than 0.1 mol/dm30.1 \text{ mol/dm}^3 CH3COOHCH_3COOH?

Solution:

HClHCl is a strong acid and dissociates completely: [H+]HCl≈0.1 mol/dm3[H^+]_{HCl} \approx 0.1 \text{ mol/dm}^3 CH3COOHCH_3COOH is a weak acid and dissociates only partially (<1%<1\%): [H+]CH3COOH<0.0013 mol/dm3[H^+]_{CH_3COOH} < 0.0013 \text{ mol/dm}^3 Since [H+]HCl>[H+]CH3COOH[H^+]_{HCl} > [H^+]_{CH_3COOH}, the pHpH of HClHCl is significantly lower.

Explanation:

Strength refers to the percentage of molecules that form ions. Because HClHCl provides more H+H^+ ions per unit volume at the same molarity, it is more acidic and has a lower pHpH.