The pH scale is a numeric scale used to indicate the concentration of hydrogen ions (H⁺) in an aqueous solution. The scale spans from 0 to 14 and is used to classify a solution as acidic, neutral, or basic based on its hydrogen ion concentration.
pH is defined as:
pH = -log₁₀[H⁺]
Here, [H⁺] denotes the hydrogen ion concentration in moles per liter within the solution.
Strong acids like HCl, HNO₃, and H₂SO₄ fully ionize in water, releasing one or more H⁺ ions per molecule. Similarly, strong alkalis such as NaOH and KOH completely dissociate in water, providing OH⁻ ions.
This complete dissociation allows for direct calculation of [H⁺] or [OH⁻] from the given molarity.
As strong acids undergo complete dissociation in water, the concentration of hydrogen ions (H⁺) is the same as the acid’s molar concentration.
HCl → H⁺ + Cl⁻
If the concentration of HCl is 0.01 M:
[H⁺] = 0.01 M
pH = -log₁₀(0.01) = 2
For 0.0001 M HNO₃:
pH = -log₁₀(0.0001) = 4
Strong bases fully ionize, so the concentration of OH⁻ is the same as the base's molarity. To calculate pH, first determine pOH:
NaOH → Na⁺ + OH⁻
Then use:
pOH = -log₁₀[OH⁻]
pH = 14 - pOH
Example: For 0.001 M NaOH
pOH = -log₁₀(0.001) = 3
pH = 14 - 3 = 11
For 0.00001 M KOH:
pOH = 5
pH = 14 - 5 = 9
These calculations are straightforward due to complete dissociation, and mastering them gives you a strong foundation for topics like titration, salt hydrolysis, and buffer solutions.