Question

The [H3O+] in a wine bottle is 3.1  10–2 M. What is the [–OH] in this wine?

Answer 1

You need to know the Kw for water at the temperature of the wine.

There is a table of Kw versus temperature here

http://www.chemguide.co.uk/physical/acidbaseeqia/kw.html

then use

[H+][OH-]=Kw

to find [OH-]