Acid and Base (pH and pOH)

Question

A 8.70⋅10^−3 M solution of HCl is made. 

What is the pH of the solution?

What is the pOH of the solution?

What is the [OH^-] in the solution?

I'm getting these marked wrong for some reason. I don't know if it is because I am doing it wrong or rounding to 3 sig figs wrong on the last one.

Answer 1

pH is the negative log of the [H⁺]. So for HCl (a strong acid), the [H⁺] = 8.7x10^-3 M

Take the negative log of that to obtain the pH

pH + pOH = 14

Once you have pH, just subtract that from 14 to obtain the pOH

[H⁺][OH^-] = Kw = 1x10^-14

[OH-] = 1x10^-14 / [H⁺] = 1x10^-14 / 8.70x10^-3 = 1.15x10^-12 M (3 sig. figs.)