Question

Which is not a way to express the hydroxide and hydronium ion concentrations in pure water at 25°C?

Answer 1

Here the word pure water means that there is nothing present in water only and only 100 % water :) .

As we know that water can dissociate producing H⁺ (H₃O⁺) and OH⁻ as shown below,
                                 H₂O   ↔   H⁺  +  OH⁻
or,
                                 H₂O + H₂O  ↔  H₃O⁺  +  OH⁻
Now,
The amount of H⁺ and OH⁻ are very small in pure water and their concentration is as,
                                 [H⁺]   =   1 × 10⁻⁷ M      (Option C is correct)

                                 [OH⁻]  =  1 × 10⁻⁷ M,

Taking antilog of [H⁺],
                                   -log [H⁺]  =  pH
     
                                  -log [1 × 10⁻⁷]  =  pH

                                        pH  =  7       (Option A is correct)
But,
Option B (i.e. 1.0 × 10⁷ M) concentration is incorrect because it is very high concentration both for H⁺ ions ( making it acidic) and for OH⁻ ions (making it basic).