To determine if an acid is strong or weak by its formula, the main way is to recognize if the acid is one of the very few known strong acids. Strong acids completely dissociate (ionize 100%) in water, while weak acids only partially dissociate.

How to tell from the formula:

• There are only seven common strong acids, which should be memorized:
  • Hydrochloric acid (HCl)
  • Hydrobromic acid (HBr)
  • Hydroiodic acid (HI)
  • Nitric acid (HNO3)
  • Sulfuric acid (H2SO4)
  • Perchloric acid (HClO4)
  • Chloric acid (HClO3)
• If the acid formula matches one of these, it is a strong acid.
• If it is not one of these known strong acids, it is considered weak.
• For acids other than these, the strength can sometimes be inferred from how completely they dissociate or from their acid dissociation constant (pKa), but that information is beyond just looking at the formula.

Thus, by formula alone, knowing this list of seven strong acids allows you to identify if the acid is strong or weak—if it’s not in the list, it is weak.

Additional notes:

• Strong acids dissociate completely in water, producing a high concentration of H+ ions.
• Weak acids dissociate partially and establish an equilibrium between undissociated acid and ions.
• The conjugate base of a strong acid is very weak and does not affect pH significantly.
• In contrast, the conjugate base of a weak acid can be relatively strong, affecting the solution's pH.

This is the simplest and most reliable way to categorize acids based on formula recognition.