Nitric acid vs Nitrous acid Difference? The formula for Nitric Acid is HNO₃, while the formula for Nitrous Acid is HNO_2. HNO₃ is a strong acid, HNO₂ is a weak acid. In other words, the pK_a value for HNO₂ is higher than that of HNO₃ because HNO₂ is a weaker acid. Note that K_a is acid dissociation constant, pKa is equal to -log₁₀k_a