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Acids and Bases - The pH Scale

Grade 9IB

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

🔑Concepts

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The pH scale is a logarithmic scale used to specify the acidity or basicity of an aqueous solution, typically ranging from 00 to 1414.

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Acids are substances that release hydrogen ions (H+H^+) in solution and have a pH value less than 77.

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Bases (or alkalis if soluble) are substances that release hydroxide ions (OH−OH^-) in solution and have a pH value greater than 77.

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A neutral solution, such as pure H2OH_2O at 25∘C25^\circ C, has a pH of exactly 77, where [H+]=[OH−][H^+] = [OH^-].

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The pH scale is logarithmic: each whole pH value below 77 is ten times more acidic than the next higher value. For example, a pH of 44 is 1010 times more acidic than a pH of 55 and 100100 times more acidic than a pH of 66.

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Indicators are chemical substances that change color depending on the pH of the solution. Common examples include Universal Indicator, Litmus paper (Red in acid, Blue in base), and Phenolphthalein.

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A neutralization reaction occurs when an acid reacts with a base to produce a salt and water: Acid+Base→Salt+WaterAcid + Base \rightarrow Salt + Water.

📐Formulae

pH=−log⁡10[H+]pH = -\log_{10}[H^+]

[H+]=10−pH[H^+] = 10^{-pH}

[H+][OH−]=1×10−14 (at 25∘C)[H^+][OH^-] = 1 \times 10^{-14} \text{ (at } 25^\circ C)

HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)

💡Examples

Problem 1:

A solution has a hydrogen ion concentration of [H+]=1×10−3 mol/dm3[H^+] = 1 \times 10^{-3} \text{ mol/dm}^3. Calculate its pH and determine if it is acidic, basic, or neutral.

Solution:

Using the formula pH=−log⁡10[H+]pH = -\log_{10}[H^+], we substitute the value: pH=−log⁡10(10−3)=3pH = -\log_{10}(10^{-3}) = 3.

Explanation:

Since the pHpH is 33, which is less than 77, the solution is acidic.

Problem 2:

If Solution A has a pH of 22 and Solution B has a pH of 55, how many times more acidic is Solution A than Solution B?

Solution:

Difference in pH units = 5−2=35 - 2 = 3. Since the scale is logarithmic (base 1010), the difference in concentration is 103=100010^3 = 1000.

Explanation:

Solution A is 10001000 times more acidic (has 10001000 times the concentration of H+H^+ ions) than Solution B.

Problem 3:

Write the balanced chemical equation for the neutralization reaction between Sulfuric acid (H2SO4H_2SO_4) and Potassium hydroxide (KOHKOH).

Solution:

H2SO4(aq)+2KOH(aq)→K2SO4(aq)+2H2O(l)H_2SO_4(aq) + 2KOH(aq) \rightarrow K_2SO_4(aq) + 2H_2O(l)

Explanation:

The acid and base react to form Potassium sulfate (the salt) and water. Two moles of KOHKOH are needed to neutralize one mole of H2SO4H_2SO_4 because sulfuric acid is diprotic (releases two H+H^+ ions).