Final answer:
Pure water at 40 °C is neutral because the product of [H3O+] and [OH−] equals the autoionization constant (Kw) for water at that temperature. Neutrality is not determined by the absolute concentrations of these ions but by their product equaling Kw.
Step-by-step explanation:
A sample of pure water at 40 °C is considered neutral even though the concentration of hydronium ions [H3O+] is 1.7 × 10−7 M because neutrality in water is based on the product of the concentrations of hydronium and hydroxide ions being equal to the autoionization constant (Kw) of water for a given temperature, not on the absolute values of these concentrations.
At 40 °C, Kw = 2.9 × 10−14, so to be neutral, [H3O+] must equal [OH−], which is also 1.7 × 10−7 M. Since the ionization of water is temperature dependent, the pH and pOH at which water is neutral will change with temperature, but as long as the product of [H3O+] and [OH−] equals Kw, the water remains neutral.