Final answer:
To calculate the hydronium ion concentration, subtract the pOH from 14 to get the pH and then use the pH to find the hydronium concentration which is 1×10⁻⁸ M.
Step-by-step explanation:
To determine the hydronium ion concentration in an aqueous solution when given the hydroxide ion concentration, we can use the pH and pOH relationship. At 25°C, the sum of pH and pOH is always 14. If the hydroxide ion concentration is 0.001 M, the pOH would be –log(0.001) which equals 3. Thus, the pH would be 14 – 3 = 11. To find the concentration of hydronium ions, we calculate 10–pH, which is 10–11 or 1×10–8 M.