MOST SEARCHED ACID STRENGTHS

STRENGTH LADDER — STRONGEST FIRST

1. 1HSbF6Fluoroantimonic acidThe strongest known acid — about 10¹⁶ times stronger than sulfuric acid-25
2. 2FSO3HFluorosulfonic acidA superacid used to make 'magic acid'-15
3. 3CF3SO3HTriflic acidOne of the strongest simple organic acids-14
4. 4HClO4Perchloric acidThe strongest common oxoacid of chlorine-10
5. 5HIHydroiodic acidThe strongest hydrohalic acid — fully ionised in water-10
6. 6HBrHydrobromic acidA strong acid, slightly weaker than HI-9
7. 7HClHydrochloric acidThe classic strong acid of the laboratory and of stomach juice-7
8. 8HBrO4Perbromic acidStrong but unstable oxoacid of bromine-4.6
9. 9H2SO4Sulfuric acidA strong diprotic acid — the most produced chemical in the world-3
10. 10H2SeO4Selenic acidStrong diprotic acid, similar to sulfuric-3
11. 11HMnO4Permanganic acidStrong acid and powerful oxidiser-2.25
12. 12H3O+Hydronium ionThe strongest acid that can exist in water-1.74
13. 13HNO3Nitric acidA strong, strongly oxidising acid-1.4
14. 14HClO3Chloric acidStrong oxoacid, weaker than perchloric-1
15. 15CF3COOHTrifluoroethanoic acid (TFA)Fluorine is even more electron-withdrawing than chlorine0.23
16. 16CCl3COOHTrichloroethanoic acidThree chlorines make it nearly as strong as a mineral acid0.66
17. 17HIO3Iodic acidA moderately strong solid oxoacid0.77
18. 18HOOC-COOHEthanedioic acid (oxalic)A strong-ish diprotic organic acid found in rhubarb1.25
19. 19CHCl2COOHDichloroethanoic acidTwo chlorines, roughly 30× stronger again1.35
20. 20H2SO3Sulfurous acidFormed when SO₂ dissolves in water — a cause of acid rain1.81
21. 21C4H4O4Maleic acidcis-butenedioic acid — the first proton is unusually strong1.9
22. 22HSO4-Hydrogensulfate ionThe second proton of sulfuric acid — only partly ionised1.99
23. 23H3PO4Phosphoric acidA weak triprotic acid found in cola drinks2.15
24. 24Fe3+Hexaaquairon(III) ion3+ metal ions make surprisingly acidic solutions2.2
25. 25HOOC-CH2-COOHPropanedioic acid (malonic)Diprotic acid used in malonic ester synthesis2.83
26. 26CH2ClCOOHChloroethanoic acidMuch stronger than ethanoic acid because of the chlorine2.87
27. 27C7H6O3Salicylic acidStronger than benzoic acid thanks to internal hydrogen bonding2.97
28. 28C4H6O6Tartaric acidDiprotic acid of grapes and baking powder2.98
29. 29C6H8O7Citric acidThe triprotic acid of lemons3.13
30. 30HFHydrofluoric acidThe only weak hydrohalic acid3.17
31. 31HNO2Nitrous acidA weak, unstable acid made in situ from NaNO₂ + HCl3.35
32. 32C9H8O4Aspirin (acetylsalicylic acid)The everyday painkiller is a weak carboxylic acid3.49
33. 33HCOOHMethanoic acid (formic)The strongest simple carboxylic acid — the sting of ants and nettles3.75
34. 34C3H6O3Lactic acidMade in muscles and in sour milk3.86
35. 35Cr3+Hexaaquachromium(III) ionAcidic green solution4
36. 36C6H8O6Ascorbic acid (vitamin C)An enol, not a carboxylic acid, yet still noticeably acidic4.1
37. 37C6H5COOHBenzoic acidA solid aromatic acid used as a food preservative4.2
38. 38C6H5NH3+Anilinium ionConjugate acid of aniline — much stronger than ammonium4.63
39. 39CH3COOHEthanoic acid (acetic)Vinegar — the reference weak acid of every syllabus4.76
40. 40C3H7COOHButanoic acidThe smell of rancid butter4.82
41. 41CH3CH2COOHPropanoic acidSlightly weaker than ethanoic acid — the longer chain pushes more electrons4.87
42. 42Al3+Hexaaquaaluminium ionWhy aluminium salts give acidic solutions5
43. 43H2CO3Carbonic acidFormed when CO₂ dissolves — controls blood and ocean pH6.35
44. 44H2SHydrogen sulfideRotten-egg gas, a very weak diprotic acid7
45. 45H2PO4-Dihydrogenphosphate ionThe second phosphoric acid proton — the key intracellular buffer7.2
46. 46HClOHypochlorous acidThe active species in bleach and swimming pools7.53
47. 47Cu2+Hexaaquacopper(II) ionOnly weakly acidic — 2+ ions are much less acidic than 3+ ions8
48. 48HBrOHypobromous acidWeaker than hypochlorous acid8.6
49. 49HCNHydrocyanic acidExtremely toxic but only a weak acid9.21
50. 50H3BO3Boric acidActs as a Lewis acid, accepting OH⁻ rather than donating H⁺9.24
51. 51NH4+Ammonium ionThe conjugate acid of ammonia — makes ammonium salts slightly acidic9.25
52. 52Fe2+Hexaaquairon(II) ionVery weakly acidic pale green solution9.5
53. 53C6H5OHPhenolAcidic enough to react with NaOH but not with Na₂CO₃9.95
54. 54HCO3-Hydrogencarbonate ionBaking soda in water — very weakly acidic, mildly basic overall10.33
55. 55HIOHypoiodous acidThe weakest of the hypohalous acids10.5
56. 56CH3NH3+Methylammonium ionConjugate acid of methylamine — weaker than ammonium10.66
57. 57Mg2+Hexaaquamagnesium ionEssentially neutral in water11.4
58. 58H2O2Hydrogen peroxideA very weak acid as well as an oxidiser11.65
59. 59HPO4 2-Hydrogenphosphate ionThe third and weakest phosphoric acid proton12.35
60. 60CH3OHMethanolMarginally more acidic than ethanol15.5
61. 61H2OWaterThe reference point — amphoteric, both acid and base15.7
62. 62C2H5OHEthanolBarely acidic — needs sodium metal, not NaOH, to deprotonate16
63. 63CH3COCH3Propanone (acetone)The alpha hydrogens are weakly acidic — the basis of enolate chemistry19.3
64. 64HC≡CHEthyne (acetylene)sp carbon makes the terminal hydrogen acidic enough for NaNH₂25
65. 65NH3Ammonia (as an acid)Only deprotonated by extremely strong bases like sodium metal38
66. 66CH4MethaneEssentially not acidic at all — the bottom of the scale50

RULES WORTH MEMORISING

• The lower the pKa, the stronger the acid — each unit is a factor of 10.
• Strong acids (HCl, HNO₃, H₂SO₄, HClO₄, HBr, HI) ionise completely; weak acids reach an equilibrium.
• Ka = 10^(−pKa); pH of a weak acid ≈ ½(pKa − log c).
• Electron-withdrawing groups (Cl, F, NO₂) strengthen an acid; alkyl groups weaken it.
• A stronger acid always has a weaker conjugate base.
• Carboxylic acids (pKa ≈ 4–5) ≫ phenols (≈10) ≫ alcohols (≈16).
• Down a group acidity rises (HF < HCl < HBr < HI); across a period it also rises (CH₄ < NH₃ < H₂O < HF).