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Acids, Bases and Salts - How Strong Are Acid Or Base Solutions

Grade 10CBSE

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

🔑Concepts

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The strength of an acid or a base is determined by the concentration of H+H^+ ions or OH−OH^- ions produced in the solution.

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A scale for measuring hydrogen ion concentration in a solution, called the pH scale, has been developed. The 'p' in pH stands for 'potenz' in German, meaning power.

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On the pH scale, we can measure pH generally from 00 (very acidic) to 1414 (very alkaline).

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Neutral solutions have a pH=7pH = 7. Values less than 77 on the pH scale represent an acidic solution, while values greater than 77 represent a basic solution.

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As the pHpH value increases from 77 to 1414, it represents an increase in OH−OH^- ion concentration in the solution, which means an increase in the strength of alkali.

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Strong acids, such as HClHCl or H2SO4H_2SO_4, give rise to more H+H^+ ions and are said to be strong acids. Acids that give less H+H^+ ions, such as CH3COOHCH_3COOH, are said to be weak acids.

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The pH of our body works within the range of 7.07.0 to 7.87.8. When the pH of rain water is less than 5.65.6, it is called acid rain.

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Tooth decay starts when the pH of the mouth is lower than 5.55.5 because the tooth enamel (calcium hydroxyapatite) gets corroded.

📐Formulae

pH=−log⁡10[H+]pH = -\log_{10}[H^+]

[H+]=10−pH[H^+] = 10^{-pH}

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

H2O(l)+H+(aq)→H3O+(aq)H_2O(l) + H^+(aq) \rightarrow H_3O^+(aq)

💡Examples

Problem 1:

You have two solutions, A and B. The pH of solution A is 66 and pH of solution B is 88. Which solution has more hydrogen ion concentration? Which of this is acidic and which one is basic?

Solution:

Solution A (pH=6pH = 6) has a higher [H+][H^+] concentration than Solution B (pH=8pH = 8). Solution A is acidic and Solution B is basic.

Explanation:

The pH scale is inversely proportional to the concentration of hydrogen ions. A lower pH value indicates a higher concentration of H+H^+ ions. Since 6<76 < 7, solution A is acidic. Since 8>78 > 7, solution B is basic.

Problem 2:

Five solutions A, B, C, D and E when tested with universal indicator showed pH as 4,1,11,74, 1, 11, 7 and 99, respectively. Which solution is: (a) neutral? (b) strongly alkaline? (c) strongly acidic? (d) weakly acidic? (e) weakly alkaline?

Solution:

(a) D (pH=7pH = 7), (b) C (pH=11pH = 11), (c) B (pH=1pH = 1), (d) A (pH=4pH = 4), (e) E (pH=9pH = 9)

Explanation:

According to the pH scale: pH=7pH = 7 is neutral. Higher values like 1111 are strongly alkaline while 99 is weakly alkaline. Lower values like 11 are strongly acidic while 44 is weakly acidic.

Problem 3:

Calculate the difference in H+H^+ ion concentration between a solution of pH=2pH = 2 and pH=3pH = 3.

Solution:

Ratio=10−210−3=10Ratio = \frac{10^{-2}}{10^{-3}} = 10

Explanation:

Using the formula [H+]=10−pH[H^+] = 10^{-pH}, for pH=2pH = 2, [H+]=10−2 mol/L[H^+] = 10^{-2} \text{ mol/L}. For pH=3pH = 3, [H+]=10−3 mol/L[H^+] = 10^{-3} \text{ mol/L}. The solution with pH=2pH = 2 has 1010 times more H+H^+ ions than the solution with pH=3pH = 3.