A particular aqueous solution at 50°C contains 3.7 * 10-4 hydronium ions. It says to calculate the hydroxide ion concentration and identify the solution as either being acidic, basic or neutral. All right, so hydronium ion is H3O+, and they want us to find OH-. The equation that connects them together is
k W = [ H 3 O + ] × [ OH - ]Our temperature is at 50°C, which means our kW value changes. If you look up above, you'll see that at 50°C kW equals 5.476 * 10-14. Plug in our number for the hydronium ion concentration. So 3.7 * 10-4 and then we just have to solve for the hydroxide ion concentration. Divide both sides by 3.7 * 10-4.
When you do that, you're going to get your hydroxide ion concentration being equal to 1.48 * 10-10 molar. Now how do we determine if it's acidic, basic or neutral solution? Well, you can see that your hydronium ion concentrations is to the -4, but your hydroxide is to the -10. Since hydronium ion concentration is greater than hydroxide ion concentration, that means that we are dealing with an acidic solution, right?
So we have both the concentration of hydroxide ion and the fact that our solution is A/C.