Question

If the [H+] in a solution is 1 × 10–1 mol/L, what is the [OH–]?

Answer 1

To answer the problem, we are given with the H+ concentration and is asked for the OH-concentration of the solution. In this case, we can determine the OH-concentration by using the equilibrium constant of water, Kw equal to 1x10-14. Hence the OH concentration is equal to 1x10-14/ 1x10-1 equal to 1x10-13 M.

Answer 2

You need to know the following equations:
pH=-log[H⁺]
pOH=-log[OH⁻]
pH+pOH=14
then you can find the pH by using pH=-log(1×10⁻¹)
pH=1
then you can find pOH by doing pOH=14-1
pOH=13
then you can find [OH] by using [OH⁻]=10^(-pOH)
[OH⁻]=1×10⁻¹³mol/L

You could also use the equation Kw=[OH⁻][H⁺] where Kw=10⁻¹⁴
10⁻¹⁴/10⁻¹=10⁻¹³mol/L

I hope this helps.  Let me know if anything is unclear.