Final answer:
The hydroxide ion concentration ([OH-]) is calculated using the ion product constant of water (Kw) and the given hydronium ion concentration ([H3O+]). By substituting the given values into the formula [OH-] = Kw / [H3O+], we find that the [OH-] is 2.857x10^-6 M.
Step-by-step explanation:
To calculate the hydroxide ion concentration ([OH⁻]) given the hydronium ion concentration ([H₃O⁺] = 3.5×10⁻⁹ M), we use the ion product constant for water (Kw) at 25°C, which is 1.0×10⁻¹⁴.
The relationship between the concentrations of [H₃O⁺] and [OH⁻] is given by the expression:
Kw = [H₃O⁺][OH⁻]
Rearranging the formula to solve for [OH⁻], we get:
[OH⁻] = Kw / [H₃O⁺]
Substituting the given value:
[OH⁻] = 1.0×10⁻¹⁴ / (3.5×10⁻⁹) = 2.857×10⁻⁶ M
Thus, the concentration of hydroxide ions in the solution is 2.857×10⁻⁶ M.