Answer is: 4·10⁻⁴ M.
pH (soft drink) = 3,4. This drink is acidic.
c(H₃O⁺) = ? - concentration of hydronium ion.
pH = -log c(H₃O⁺) ⇒ c(H₃O⁺) = 10∧-pH.
c(H₃O⁺) = 10⁻³·⁴.
c(H₃O⁺) = 4·10⁻⁴ M(mol/dm³).
pH (potential of hydrogen) is a numeric scale used to define the acidity or basicity of an aqueous solution.