Question

which statement best describes the ph of pure water?
∘ it is neutral because the concentration of hydronium ions equals that of hydroxide ions.
∘ it is neutral because the pure liquid contains neither hydronium ions nor hydroxide ions.
∘ it is acidic because it has a hydronium ion concentration of 1.0×10⁻⁷ m.
∘ it is basic because it has a hydroxide ion concentration of 1.0×10⁻⁷ m.

Explanation:

• For the second option: Pure water undergoes auto - ionization, producing $\ce{H3O+}$ (hydronium) and $\ce{OH-}$ (hydroxide) ions, so it does contain these ions.
• For the third option: A hydronium ion concentration of $1.0\times 10^{-7}\ M$ at $25^{\circ}C$ corresponds to a pH of 7, which is neutral, not acidic.
• For the fourth option: A hydroxide ion concentration of $1.0\times 10^{-7}\ M$ at $25^{\circ}C$ also corresponds to a pH of 7 (since $K_w = [\ce{H3O+}][\ce{OH-}]=1.0\times 10^{-14}$ at $25^{\circ}C$, so $[\ce{H3O+}]=[\ce{OH-}]=1.0\times 10^{-7}\ M$), and a solution with equal concentrations of hydronium and hydroxide ions is neutral, not basic.
• For the first option: In pure water, the auto - ionization reaction $\ce{H2O + H2O

ightleftharpoons H3O+ + OH-}$ occurs. At $25^{\circ}C$, the concentrations of $\ce{H3O+}$ and $\ce{OH-}$ are both $1.0\times 10^{-7}\ M$. When the concentration of hydronium ions equals the concentration of hydroxide ions, the solution is neutral.

Answer:

It is neutral because the concentration of hydronium ions equals that of hydroxide ions.