Final answer:
Solution A with a pH of 3.5 is 10,000,000 times more acidic than solution B with a pH of 10.5.
Step-by-step explanation:
The pH scale is logarithmic, which means that a difference of 1 pH unit represents a difference of a factor of 10 in hydronium ion concentration. Solution A has a pH of 3.5, while solution B has a pH of 10.5. The difference in pH between the two solutions is 10.5 - 3.5 = 7 units. Since each unit corresponds to a factor of 10, the hydronium ion concentration of solution A is 10^7 times greater than that of solution B. Therefore, solution A is 10,000,000 times more acidic than solution B.