In pure water at 25°C, the concentration of hydronium ions (H3O+) is \(1.0 \times 10^{-7} , \text{mol/L}\). Therefore, the correct option is:
1.0 × 10^-7 mol/L
or
1.0 x 10 -7 mol/L
Both options represent the same concentration, so either of those would be correct.