X = - log10 H+] What does X stand for in the expression above?

Step 1: Analyze the question The expression X = -log₁₀[H⁺] is the definition of pH. The question asks which option best describes what pH stands for.

Step 2: Explain the concept pH measures the concentration of hydrogen ions in a solution, and so how acidic or basic the solution is.

• pH < 7: acidic (high [H⁺])
• pH = 7: neutral
• pH > 7: basic (low [H⁺])

Step 3: Check each option

• A. Degree of acidity: pH expresses how acidic or alkaline a solution is, based on [H⁺]. This fits.
• B. Enthalpy change: this is ΔH, heat change in a reaction. It has nothing to do with [H⁺].
• C. Equilibrium constant: this is K, a ratio of product and reactant concentrations. It is not a negative log of [H⁺].
• D. Ionic product of water: this is Kw = [H⁺][OH⁻] = 1 × 10⁻¹⁴ at 25°C. It is a product of concentrations, not -log[H⁺]. (The related quantity pKw = -log Kw = 14 is a different thing.)
• E. Solubility product: this is Ksp, for sparingly soluble salts. It does not apply here.

Step 4: Answer X is the pH, which is a measure of the degree of acidity of a solution.

Step 5: Exam tip Remember "p" means "negative log of". So pH = -log[H⁺], pOH = -log[OH⁻], and pH + pOH = 14 at 25°C. If you see -log of [H⁺] in an option question, think pH, which is acidity.