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Chemistry - Acids, bases, and salts (pH scale and preparation of salts)

Grade 9IGCSE

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

🔑Concepts

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The pH scale is a numerical scale from 00 to 1414 used to specify the acidity or alkalinity of an aqueous solution. A pH<7pH < 7 is acidic, pH=7pH = 7 is neutral, and pH>7pH > 7 is alkaline.

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Acids are proton (H+H^+) donors. Strong acids like HClHCl and H2SO4H_2SO_4 fully dissociate in water, while weak acids like CH3COOHCH_3COOH only partially dissociate.

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Bases are proton (H+H^+) acceptors. Alkalis are soluble bases that produce hydroxide ions (OH−OH^-) in solution.

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Neutralization is the reaction between H+H^+ ions from an acid and OH−OH^- ions from a base to form water (H2OH_2O).

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To prepare a soluble salt from an acid and an insoluble base (like CuOCuO), the 'Excess Base' method is used: add excess base to the acid, filter the unreacted solid, and evaporate the filtrate to crystallize the salt.

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To prepare a soluble salt from an acid and a soluble base (alkali), 'Titration' is required to determine the exact stoichiometric volumes needed.

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To prepare an insoluble salt (like BaSO4BaSO_4), the 'Precipitation' method is used by mixing two soluble salt solutions together.

📐Formulae

ightarrow Salt + H_2(g)$$
ightarrow Salt + H_2O(l)$$
ightarrow Salt + H_2O(l) + CO_2(g)$$
ightarrow H_2O_{(l)}$$
ightarrow BaSO_{4(s)} + 2NaCl_{(aq)}$$

💡Examples

Problem 1:

Describe how to prepare dry crystals of Copper(II) Sulfate (CuSO4CuSO_4) starting from Copper(II) Oxide (CuOCuO) and Sulfuric Acid (H2SO4H_2SO_4).

Solution:

  1. Add excess CuOCuO to a fixed volume of dilute H2SO4H_2SO_4 while warming. 2. Filter the mixture to remove the unreacted CuOCuO. 3. Heat the filtrate (CuSO4CuSO_4 solution) until the crystallization point. 4. Allow to cool and dry the resulting crystals.

Explanation:

This follows the 'Insoluble Base + Acid' route. The reaction is: CuO(s)+H2SO4(aq)ightarrowCuSO4(aq)+H2O(l)CuO_{(s)} + H_2SO_{4(aq)} ightarrow CuSO_{4(aq)} + H_2O_{(l)}. Excess base ensures all acid is neutralized.

Problem 2:

Predict the pH and the ion concentration change when NaOHNaOH is added to a solution of HClHCl.

Solution:

The pHpH will increase towards 77. The concentration of H+H^+ ions decreases as they react with OH−OH^- to form H2OH_2O.

Explanation:

The neutralization reaction is H++Cl−+Na++OH−ightarrowNa++Cl−+H2OH^+ + Cl^- + Na^+ + OH^- ightarrow Na^+ + Cl^- + H_2O. As H+H^+ ions are consumed, the solution becomes less acidic.