Final answer:
The concentration of hydronium ions ([H3O+]) in a solution with a pH of 5.5 is 3 × 10^-6 M, expressed to one significant figure.
Step-by-step explanation:
To find the concentration of hydronium ions ([H3O+]) for a solution with a pH of 5.5, we use the equation pH = -log [H3O+]. Rearranging this equation we get [H3O+] = 10-pH. Substituting the given pH value into this expression, we get [H3O+] = 10-5.5, which equals approximately 3.2 × 10-6 M. However, since the pH was given to two significant figures, the answer should be given to one significant figure, which would be 3 × 10-6 M. Note that significant figures from the pH value determine the accuracy of our final answer.