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pH & Hydrogen Ions Simplified Revision Notes

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5.5.2 pH & Hydrogen Ions

Understanding the pH Scale

The concentration of hydrogen ions ([H+][\text{H}^+]) in aqueous solutions varies widely, so chemists use the pH scale to express these concentrations more conveniently.

The pH scale is logarithmic, meaning each whole number change on the pH scale corresponds to a tenfold change in [H+][\text{H}^+].

Definition of pH:

pH=log10[H+]\text{pH} = -\log_{10} [\text{H}^+]

Conversely, if you know the pH, you can find [H+][\text{H}^+]

[H+]=10pH[\text{H}^+] = 10^{-\text{pH}}

pH Scale Values:

  • Neutral solutions (at 25°C): pH = 7.00
  • Acidic solutions: pH < 7.00
  • Alkaline solutions: pH > 7.00

Calculating pH for Strong Acids

In solutions of strong acids (e.g., HClHCl, H2SO4H₂SO₄), the acid fully dissociates, so the concentration of [H+][\text{H}^+] is directly related to the concentration of the acid.

infoNote

Example Calculation: pH of Sulfuric Acid Solution Calculate the pH of a 1 mol dm3^{-3} solution of sulfuric acid (H2SO4\text{H}_2\text{SO}_4).


Step 1: Identify Proton Contribution:

H2SO4\text{H}_2\text{SO}_4 is a diprotic acid, meaning it releases two H+\text{H}^+ ions per molecule.

For each mole of H2SO4\text{H}_2\text{SO}_4, there are 2 moles of H+\text{H}^+.

H2SO42H++SO42\text{H}_2\text{SO}_4 \rightarrow 2\text{H}^+ + \text{SO}_4^{2-}

Step 2: Calculate [H+][\text{H}^+]:

Multiply the concentration of H2SO4\text{H}_2\text{SO}_4 by 2.

[H+]=2×1=2mol dm3[\text{H}^+] = 2 \times 1 = 2 \, \text{mol dm}^{-3}

Step 3: Calculate pH:

pH=log10(2):success[0.30]\text{pH} = -\log_{10}(2) \approx :success[-0.30]

Calculating [H+][\text{H}^+] from pH

Sometimes, you know the pH and need to find the concentration of the acid.

infoNote

Example Calculation: Finding Concentration of H2SO4\text{H}_2\text{SO}_4 from pH Calculate the concentration of sulfuric acid with a pH of 1.00.


Step 1: Calculate [H+][\text{H}^+]:

[H+]=10pH=101=0.1mol dm3[\text{H}^+] = 10^{-\text{pH}} = 10^{-1} = 0.1 \, \text{mol dm}^{-3}

Step 2: Account for Diprotic Nature:

Since sulfuric acid is diprotic, [H+][\text{H}^+] is twice the concentration of H2SO4\text{H}_2\text{SO}_4.

Concentration of H2SO4=0.12=:success[0.05]mol dm3\text{Concentration of } \text{H}_2\text{SO}_4 = \frac{0.1}{2} = :success[0.05] \, \text{mol dm}^{-3}
infoNote

Summary

  • pH Calculation: pH=log10[H+]\text{pH} = -\log_{10} [\text{H}^+]
  • [H+][\text{H}^+] Calculation: [H+]=10pH[\text{H}^+] = 10^{-\text{pH}}
  • Strong Acids: Fully dissociate, so [H+][\text{H}^+] often equals or is a multiple of the acid's concentration depending on its proticity.
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