Question

if the [H+] in a solution is 1*10^-1 mol/L, what is the [OH]?

Answer 1

K(w)=[OH⁻][H⁺]  (K(w)=1×10⁻¹⁴)

[OH⁻]=K(w)/[H⁺]
[OH⁻]=(1×10⁻¹⁴)/0.1
[OH⁻]=1×10⁻¹³M

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