pH = -log[H₃O⁺(aq)]
here the pH is given as 2.40.
Hence,
  2.40               = -log[H₃O⁺(aq)]
log[H₃O⁺(aq)]  = -2.40
[H₃O⁺(aq)]       = 3.98 x 10⁻³ M
                        = 4.0 x 10⁻³ M
Hence, the hydronium ion concentration in juice is 4.0 x 10⁻³ M.
Since, the pH is less than 7, the juice is acidic.