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Chemistry - Acids, Bases, and pH Scales

Grade 10IB

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

🔑Concepts

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Acids are proton (H+H^+) donors and produce hydrogen ions (H+H^+) in aqueous solutions, resulting in a pH<7pH < 7.

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Bases are proton (H+H^+) acceptors; soluble bases are called alkalis and produce hydroxide ions (OH−OH^-) in aqueous solutions, resulting in a pH>7pH > 7.

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The pHpH scale is a logarithmic scale ranging from 00 to 1414, where each unit change represents a ten-fold change in the concentration of hydrogen ions [H+][H^+].

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Strong acids (e.g., HClHCl, HNO3HNO_3, H2SO4H_2SO_4) completely dissociate in water, while weak acids (e.g., CH3COOHCH_3COOH) only partially dissociate.

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Neutralization is a chemical reaction between an acid and a base that produces a salt and water: H(aq)++OH(aq)−→H2O(l)H^+_{(aq)} + OH^-_{(aq)} \rightarrow H_2O_{(l)}.

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Indicators such as Phenolphthalein, Litmus, and Universal Indicator change color to signify the pHpH or equivalence point of a solution.

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Acids react with metal carbonates to produce salt, water, and carbon dioxide gas: 2HCl+Na2CO3→2NaCl+H2O+CO22HCl + Na_2CO_3 \rightarrow 2NaCl + H_2O + CO_2.

📐Formulae

pH=−log⁡10[H+]pH = -\log_{10}[H^+]

[H+]=10−pH[H^+] = 10^{-pH}

Kw=[H+][OH−]=1.0×10−14 at 25∘CK_w = [H^+][OH^-] = 1.0 \times 10^{-14} \text{ at } 25^\circ C

Acid+Base→Salt+Water\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}

Acid+Metal→Salt+H2(g)\text{Acid} + \text{Metal} \rightarrow \text{Salt} + H_2(g)

💡Examples

Problem 1:

Calculate the pHpH of a 0.001 mol/dm30.001\text{ mol/dm}^3 solution of nitric acid (HNO3HNO_3), assuming it is a strong acid that dissociates completely.

Solution:

pH=3pH = 3

Explanation:

Since HNO3HNO_3 is a strong acid, the concentration of hydrogen ions [H+][H^+] is equal to the concentration of the acid. Therefore, [H+]=0.001 M=10−3 M[H^+] = 0.001\text{ M} = 10^{-3}\text{ M}. Using the formula pH=−log⁡10(10−3)pH = -\log_{10}(10^{-3}), we get pH=3pH = 3.

Problem 2:

Determine the concentration of H+H^+ ions in a solution with a pHpH of 5.05.0.

Solution:

[H+]=1×10−5 mol/dm3[H^+] = 1 \times 10^{-5}\text{ mol/dm}^3

Explanation:

Using the inverse pHpH formula [H+]=10−pH[H^+] = 10^{-pH}, we substitute the value of pHpH: [H+]=10−5[H^+] = 10^{-5}. This represents the molarity of the hydrogen ions in the solution.

Problem 3:

Predict the products and write the balanced chemical equation for the reaction between hydrochloric acid (HClHCl) and magnesium hydroxide (Mg(OH)2Mg(OH)_2).

Solution:

2HCl(aq)+Mg(OH)2(aq)→MgCl2(aq)+2H2O(l)2HCl_{(aq)} + Mg(OH)_{2(aq)} \rightarrow MgCl_{2(aq)} + 2H_2O_{(l)}

Explanation:

This is a neutralization reaction where an acid reacts with a base to form a salt (Magnesium Chloride) and water. Two moles of HClHCl are required to neutralize one mole of Mg(OH)2Mg(OH)_2 because Magnesium has a 2+2+ charge.

Acids, Bases, and pH Scales - Revision Notes & Key Formulas | IB Grade 10 Science