What is [OH-] in a solution that has [H3O+] = 2 x 10-4 ?

The OH- in a solution which has a H3O+ of 2x10^-4 M is 5x10^-11 M. This is because the pOH of that solution is 10.3 since it has a pH of 3.7.

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because natural gas is colorless, odorless, and tasteless, distributors add mercaptan (a chemical that smells like sulfur) to give natural gas a distinct unpleasant odor (it smells like rotten eggs). they locate the types of rock that are likely to contain natural gas deposits.