Calculate the OH- concentration

Question

An aqueous solution at  25 C has a H₂O⁺ concentration of 2. x 10^-7 M. Calculate the OH^- concentration. Be sure your answer has 1 significant digits. 

Answer 1

Remember that H3O+ and OH- concentrations are connected to each other: [H3O+]*[OH-]=1 x 10-14M

Since we are given the H3O+ concentration (2 x 10-7), we can plug this into our equation to calculate OH- concentration:

[2 x 10-7]*[OH-]=1 x 10-14

[OH-]=(1 x 10-14)/(2 x 10-7)

[OH-]=5 x 10-8 M