Which of these solutions is acidic?

Option a is a solution acidic ([OH⁻] = 7.0x10⁻⁹ M).

Explanation:

To know if a solution is acidic we need to calculate the pH and it must be lower than 7. A value of pH equal to 7 is a neutral solution and a solution with a pH value higher than 7 is a basic solution.

a. For the [OH⁻] = 7.0x10⁻⁹ M we have:

Now, the pH is:

This solution is acidic (pH < 7)

b. [H₃O⁺] = 8.5x10⁻⁸ M

This is not an acidic solution. Is a neutral one (pH around to 7).

c. [OH⁻] = 2.5x10⁻⁶ M

Then, the pH is:

Hence, this is not an acidic solution. It is basic (pH > 7).    

Therefore, option a is a solution acidic ([OH⁻] = 7.0x10⁻⁹ M).

I hope it helps you!