What are Acid-Base Properties and pH?
Acids donate protons (H⁺ ions), bases accept them. The pH scale (0–14) measures hydrogen ion concentration: pH < 7 is acidic, pH 7 is neutral, pH > 7 is basic. Understanding acid-base properties is central to chemistry.
Acids release H⁺ ions; bases accept them. pH = −log[H⁺]; lower pH means more acidic. Buffer solutions resist pH change by containing a weak acid–base conjugate pair.
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Step-by-step worked examples
If [H⁺] = 1×10⁻³ M, what is the pH?
pH = −log₁₀[H⁺] = −log₁₀(1×10⁻³) pH = −(−3) = 3
A solution has pH 9. Is it acidic or basic?
pH > 7 means basic The solution is basic (alkaline) Common examples: household ammonia, baking soda solution
If pH = 2, calculate [H⁺].
pH = −log₁₀[H⁺] 2 = −log₁₀[H⁺] log₁₀[H⁺] = −2 [H⁺] = 10⁻² = 0.01 M
Flashcards
Quick quiz
Q1.If [H⁺] = 1×10⁻⁵ M, the pH is:
Q2.pH < 7 indicates:
Q3.A buffer resists:
Q4.Which is a strong acid?
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Common mistakes
pH measures absolute acidity. — Correct: pH measures hydrogen ion concentration logarithmically; it's a scale, not absolute quantity.
Neutral always means pH 7. — Correct: Neutral means [H⁺] = [OH⁻]; at 25°C this is pH 7, but at other temperatures it differs.
A buffer only works with acids. — Correct: A buffer resists pH change by containing a conjugate acid–base pair; it works for both added acid and added base.
Strong acids are more dangerous than weak acids. — Correct: Strength (ionization%) and danger are related but not identical; concentrated weak acids can be as hazardous as dilute strong acids.
FAQ
What is the pH formula?
pH = −log₁₀[H⁺], where [H⁺] is the hydrogen ion concentration in mol/L.
How do buffers work?
A buffer contains a weak acid and its conjugate base. If H⁺ is added, the base removes it; if OH⁻ is added, the acid releases H⁺.
Is pH always between 0 and 14?
At 25°C in aqueous solution, yes; at other temperatures, the neutral pH shifts. Extremely concentrated solutions can go outside this range.
What is the difference between strong and weak acids?
Strong acids ionize nearly 100% (e.g., HCl, H₂SO₄); weak acids ionize partially (e.g., acetic acid ~1%).