Which formula is used to indicate the degree of acidity of an acid?

Analyze the question: It asks for the formula used to indicate the degree of acidity of an acid, meaning a measure of how acidic a solution is.

Concept: Acidity depends on the concentration of hydrogen ions, [H⁺], in solution. Because [H⁺] values are often tiny (such as 0.0000001 mol/dm³), chemists use the pH scale to express them as manageable numbers:

pH = −log₁₀[H⁺]

• pH < 7: acidic (the lower the pH, the stronger the acidity)
• pH = 7: neutral
• pH > 7: alkaline

Checking each option:

• A. This is not a valid formula for [H⁺]. It resembles a mix-up of the formula for molar concentration.
• B. This is not a recognised formula for acidity.
• C. Kw = [H⁺][OH⁻] is the ionic product of water. It is a constant (1.0 × 10⁻¹⁴ at 25 °C) and does not indicate the acidity of a particular solution.
• D. pH = −log₁₀[H⁺] is the standard measure of the degree of acidity of a solution. ✔
• E. pKa = −log₁₀Ka indicates the strength of an acid (how far it dissociates), but not the acidity of a given solution, which depends on concentration as well. pH is the better answer to "degree of acidity."

Tip: Remember "pH = power of Hydrogen." If a question asks about how acidic or alkaline a solution is, think pH. If it asks about the strength of an acid (dissociation), think Ka or pKa.