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Acids, Bases and Salts - The pH scale and indicators

Grade 11IGCSEChemistry

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

🔑Concepts

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The pHpH scale is a numerical scale ranging from 00 to 1414 used to specify the acidity or alkalinity of an aqueous solution.

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A solution with pH<7pH < 7 is acidic, pH=7pH = 7 is neutral (e.g., pure water), and pH>7pH > 7 is alkaline.

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Acids are proton donors that release hydrogen ions (H+H^+) in water. The concentration of these ions, denoted as [H+][H^+], determines the pHpH.

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Strong acids, such as HClHCl and H2SO4H_2SO_4, dissociate completely in aqueous solutions, whereas weak acids, such as CH3COOHCH_3COOH, only partially dissociate.

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Indicators are chemical substances that change color at specific pHpH levels. Common indicators include Litmus (Red in acid, Blue in alkali), Phenolphthalein (Colorless in acid, Pink in alkali), and Methyl Orange (Red in acid, Yellow in alkali).

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Universal Indicator is a mixture of several indicators that displays a continuous range of colors (ROYGBIV) to provide an approximate pHpH value.

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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).

📐Formulae

pH=−log⁡10[H+]pH = -\log_{10}[H^+]

H(aq)++OH(aq)−→H2O(l)H^+_{(aq)} + OH^-_{(aq)} \rightarrow H_2O_{(l)}

[H+]=10−pH[H^+] = 10^{-pH}

HCl(aq)→H(aq)++Cl(aq)−HCl_{(aq)} \rightarrow H^+_{(aq)} + Cl^-_{(aq)}

💡Examples

Problem 1:

Calculate the pHpH of a solution where the hydrogen ion concentration [H+][H^+] is 0.01 mol/dm30.01\text{ mol/dm}^3.

Solution:

pH=2pH = 2

Explanation:

Using the formula pH=−log⁡10[H+]pH = -\log_{10}[H^+], we substitute the value: pH=−log⁡10(10−2)pH = -\log_{10}(10^{-2}). This results in a pHpH of 22, indicating a strong acid.

Problem 2:

A student adds a few drops of Phenolphthalein to a flask containing NaOHNaOH. What color change occurs if the student then titrates it with excess HClHCl?

Solution:

Pink to Colorless

Explanation:

In the presence of the base NaOHNaOH, Phenolphthalein is initially pink. As HClHCl is added, it neutralizes the base. Once the pHpH drops below 8.38.3 (the acidic side), the indicator becomes colorless.

Problem 3:

Identify the ionic equation for the reaction between HNO3HNO_3 and KOHKOH.

Solution:

H(aq)++OH(aq)−→H2O(l)H^+_{(aq)} + OH^-_{(aq)} \rightarrow H_2O_{(l)}

Explanation:

In a neutralization reaction between a strong acid and a strong base, the spectator ions (K+K^+ and NO3−NO_3^-) are removed, leaving the formation of water from hydrogen and hydroxide ions.

The pH scale and indicators - Revision Notes & Key Formulas | IGCSE Grade 11 Chemistry