pH of strong acid and alkali | Part-1

Calculating the pH of Strong Acids and Bases

The term pH refers to a logarithmic scale that indicates how acidic or alkaline a water-based solution is. It is mathematically expressed as:

pH = -log10[H+]

Where [H⁺] is the concentration of hydrogen ions in mol/L.

• pH < 7: Acidic solution
• pH = 7: Neutral solution
• pH > 7: Basic (alkaline) solution

Strong Acids and Strong Alkalis

Strong acids such as HCl, HNO₃, and H₂SO₄ fully ionize in aqueous solutions, producing hydrogen ions (H⁺).
Strong bases like NaOH and KOH also undergo complete dissociation in water, releasing hydroxide ions (OH⁻).

pH of a Strong Acid

For a strong acid like HCl:

HCl → H⁺ + Cl⁻

If the concentration of HCl is C M, then [H⁺] = C

pH = -log10[C]

Examples:

• For 0.1 M HCl: pH = –log(0.1) = 1
• For 0.001 M HNO₃: pH = –log(0.001) = 3

pH of a Strong Alkali

For a strong base like NaOH:

NaOH → Na⁺ + OH⁻

First calculate pOH:

pOH = –log10[OH⁻]

Then use the formula:

pH = 14 – pOH

Examples:

• For 0.01 M NaOH: pOH = 2 → pH = 14 – 2 = 12
• For 0.0001 M KOH: pOH = 4 → pH = 14 – 4 = 10

Key Points for JEE Main

• Strong acids and bases fully ionize when dissolved in water.
• To calculate pH: apply pH = –log[H⁺] for acids, and use pH = 14 – pOH for bases.
• Be confident with logarithmic calculations, such as log(1/100) = –2, and similar properties.
• Know the difference between strong and weak electrolytes.

These calculations are direct applications of the definition of pH and the complete dissociation property of strong electrolytes.