Final answer:
The pH of solution a is 7.00 as it's a neutral solution with [OH-] = 1.0 × 10^-7 M. The pH of solution b cannot be determined without the exact value of x.
Step-by-step explanation:
Part a:
To determine the pH of a solution where the concentration of hydroxide ions [OH-] is 1.0 × 10^-7 M,
we can use the fact that the product of the concentrations of hydrogen ions [H3O+] and hydroxide ions [OH-] in water at 25 °C is always 1.0 × 10^-14.
Thus, for a neutral solution where [OH-] = 1.0 × 10^-7 M, [H3O+] will also be 1.0 × 10^-7 M.
To find the pH, we take the negative logarithm of the hydronium ion concentration:
pH = -log(1.0 × 10^-7) = 7.00
Part b:
We cannot determine the pH without the actual value of x for the hydronium ion concentration [H3O+] = 4.2 × 10^-x M. Once x is known, the pH is calculated using the formula:
pH = -log([H3O+])
In summary, the pH of solution a is 7.00, but we need the value of x to calculate the pH for solution b.