At 10 degrees Celcius, the water ionization constant, Kw is 2.9 x 10^-15. What is the H3O+ concentration in...

neutral water at this temperature?

a. 2.9 x 10^-15 M
b. 1.4 x 10^-15 M
c. 3.8 x 10^-8 M
d. 5.4 x 10^-8 M
e. 1.0 x 10^-7 M

Please help =]! Be nice if you showed full working Cheers!

Kw = 2.9 x 10^-15 = [H3O+][OH-]
but in a neutral solution, the concentration of the H3O+ and OH- are the same, so each is equal to the square root of the Kw.

[H3O+] = the square root of (2.9 x 10^-15) = 5.4 x 10^-8 and the answer is d