Quadratic Equation?

Question

Hello,

I have a quadratic formula:

[OH-]2 - (8.51 x 10^-8) - (1 x 10^-14) = 0

(the first bit there is squared, and not a number 2)

I also know that the answer for [OH-], the hydroxide ion is 6.61 x 10^-8 (i.e. i know the answer already!)

The problem i am having is trying to go through the working to end up with that answer. I did post yesterday, but i wrote out the equation wrong, so i have posted again. Thanks

the problem i am trying to work out is in the link below

http://www.chemtutor.com/xacid.htm

its on about page 4 , and gives the quadratic equation in [OH-] and then the answer 6.6115 x 10^-8 (the boric acid problem)

Answer 1

Isolate the [OH-]^2 term:
[OH-]^2 = [8.51 x 10^(-8)] + [1 x 10^(-14)]

Take the square root of the sum.

Edit: Check stated formula, again.

Answer 2

[OH-]^2 = [8.51 x 10^(-8)] + [1 x 10^(-14)]
[OH-]^2 = 0.00000008510001
OH- = +-0.0002917190600560751841118224735244

(by the way, this is NOT a quadratic equation.)

Answer 3

What not just square root the following ((8.51 x 10^-8) + (1 x 10^-14))

Answer 4

let (OH-) = X
X^2 = 8.51x10^(-8) + (1 x 10^(-14) )
X^2 = 10(^-14)(8.51x10^6 +1)
X^2 = 10(^-14)(8.51x10^6) 1 may be neglected
X^2 = 8.51 x 10^(-8)
X= 2.92 x 10^(-4)

Answer 5

Please check your numbers again
OH- = appx. 2.92 x 10^(-4)