What is [OH−] in an aqueous solution at 298 K in which [H+] = 5.35 10-3 M.

298 K is just 25 ^oC, or "room temperature".

 

[H⁺] = 5.35 * 10^-3 M 

 

 

We know that:

 

[H⁺][OH^-] = 10^-14 M²

 

[OH^-] = 10^-14 M²/[H⁺] ≈ 1.87 * 10^-12 M