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Strong Acids and Bases — Complete Ionization Reference

Name Formula Type Ionization Reaction Ka or Kb Common Use
Hydrochloric acidHClStrong acidHCl → H⁺ + Cl⁻~10⁶Stomach acid, metal cleaning, lab reagent
Hydrobromic acidHBrStrong acidHBr → H⁺ + Br⁻~10⁹Organic synthesis, pharmaceutical intermediates
Hydroiodic acidHIStrong acidHI → H⁺ + I⁻~10¹°Reducing agent, organic synthesis
Nitric acidHNO₃Strong acidHNO₃ → H⁺ + NO₃⁻~24Fertilizers, explosives, metal etching
Sulfuric acid (1st ionization)H₂SO₄Strong acidH₂SO₄ → H⁺ + HSO₄⁻~10³Car batteries, industrial chemical, dehydrating agent
Sulfuric acid (2nd ionization)HSO₄⁻Weak acidHSO₄⁻ ⇌ H⁺ + SO₄²⁻0.012 (Ka₂)Second proton only partially ionizes
Perchloric acidHClO₄Strong acidHClO₄ → H⁺ + ClO₄⁻~10¹°Analytical chemistry, rocket propellant oxidizer
Chloric acidHClO₃Strong acidHClO₃ → H⁺ + ClO₃⁻~10³Oxidizing agent, chlorate production
Lithium hydroxideLiOHStrong baseLiOH → Li⁺ + OH⁻Fully ionizedCO₂ scrubbing, lithium grease
Sodium hydroxideNaOHStrong baseNaOH → Na⁺ + OH⁻Fully ionizedSoap making, drain cleaner, lab reagent
Potassium hydroxideKOHStrong baseKOH → K⁺ + OH⁻Fully ionizedSoft soap, electrolyte in batteries
Rubidium hydroxideRbOHStrong baseRbOH → Rb⁺ + OH⁻Fully ionizedResearch chemical
Cesium hydroxideCsOHStrong baseCsOH → Cs⁺ + OH⁻Fully ionizedResearch chemical, strongest known base in aqueous solution
Calcium hydroxideCa(OH)₂Strong baseCa(OH)₂ → Ca²⁺ + 2OH⁻Fully ionizedLimewater, water treatment, mortar
Strontium hydroxideSr(OH)₂Strong baseSr(OH)₂ → Sr²⁺ + 2OH⁻Fully ionizedSugar refining, plastic stabilizer
Barium hydroxideBa(OH)₂Strong baseBa(OH)₂ → Ba²⁺ + 2OH⁻Fully ionizedTitration standard, analytical chemistry

Two caveats matter. First, H₂SO₄ is strong only in its first ionization (H₂SO₄ → H⁺ + HSO₄⁻); the second step (HSO₄⁻ ⇌ H⁺ + SO₄²⁻) has Ka₂ = 0.012 and is a weak-acid equilibrium. For sulfuric acid below about 0.5 M you can usually approximate full diprotic dissociation, but not at higher concentrations. Second, 'strong' refers to degree of ionization, not solubility. Ca(OH)₂ is a strong base but only dissolves to about 0.020 M at 25 °C — saturated limewater hits pH ~12.4 because the small dissolved fraction is fully ionized. The strong-acid list is canonical in Zumdahl's Chemistry and Silberberg's Chemistry.

Frequently Asked Questions

How many strong acids are there to memorize?
Most general chemistry textbooks list seven: HCl, HBr, HI, HNO₃, H₂SO₄ (first ionization only), HClO₄, and HClO₃. Some omit HClO₃ and call it six. A clean way to remember the set: three hydrohalic acids (HCl, HBr, HI — note HF is weak because of its strong H–F bond), plus three or four oxoacids (HNO₃, H₂SO₄, HClO₄, and sometimes HClO₃). Anything not on this list — including HF, acetic acid, phosphoric acid, and carbonic acid — is weak and needs a Ka equilibrium.
Why is HF a weak acid while HCl is strong?
Electronegativity isn't the dominant factor for hydrohalic acid strength — bond strength is. The H–F bond is 568 kJ/mol, far stronger than H–Cl at 431 kJ/mol or H–I at 297 kJ/mol. That high bond enthalpy makes HF dissociation thermodynamically unfavorable enough to keep its Ka at 6.8 × 10⁻⁴ (a weak acid). Solvation of F⁻ also costs more energy than for the larger halides because the small ion holds water tightly. Down the group, bond strength drops and acid strength climbs: HF (weak) < HCl < HBr < HI.
Can a strong base have limited solubility?
Yes — and Ca(OH)₂ is the canonical example. 'Strong' describes how completely the dissolved portion ionizes, not how much actually goes into solution. Ca(OH)₂ has a solubility of about 1.5 g/L (0.020 M) at 25 °C, but the fraction that dissolves dissociates completely into Ca²⁺ and 2OH⁻. Saturated limewater therefore reaches pH ~12.4, consistent with [OH⁻] ≈ 0.040 M. Sr(OH)₂ and Ba(OH)₂ behave the same way; Ba(OH)₂ is the most soluble and is often used as a titration standard.