Acid and Base (pH and pOH)

Question

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

What is the pOH?

What is the pH?

What is the [H⁺] in the solution (Sci Not using 3 sig figs)?

I keep getting these types of questions wrong and I can't figure out why.

Answer 1

NaOH is a strong base that will completely dissociate in water (Na+ and OH-). Therefore, the first thing to do is to calculate the concentration of OH-, which in the case of NaOH, is the same as the concentration of the base (0.0087 M). Then, we can calculate the pOH which is the -log[OH].

pOH = -log0.0087 = 2.06

Then we can calculate pH from pH + pOH = 14

pH = 14-2.06 =11.94

Since pH is the -log[H+], we can now figure out [H+] = 10^-pH

[H+] = 10^-11.94 = 1.148 x 10^-12