Question

WHAT IS THE [OH-] OF A SOLUTION WITH A PH OF 9.0?

A)1 x 10-5 M

B) 1 x 10-9 M

C) 1 x 10-4 M

D) 1 x 10-7 M

E) NONE

Answer 1

pH = 9

Using pH we can calculate the concentration of [H⁺] ions using the formula

pH = - log [H⁺]

9 = - log [H⁺]

Taking minus to the left side, we get

-9 = log [H⁺]

[H⁺] = 10^-9 M

We know that at 25 degree centrigrade, K_w of water is 1 x 10^-14 M²

K_w (Ionic Product of water) = [H⁺][OH^-] = 1 x 10^-14 M²

Substituting the value of  [H⁺] we can find the concentration of hydroxide ions ([OH^-]

[OH^-] = 1 x 10^-14 / [H⁺] = 1 x 10^-14 / 1 x 10^-9 = 10^-5

[OH^-] = 1 x 10^-5 M

Answer A