# Why is finding [H+] when given [OH-]= 3.57e-5 M wrong when I use -log[OH-]?

Question/Problem

I know that [H+][OH-]=$10^{-14}$

And the problem asks to find out the concentration of H+ when the concentration of OH- is $3.57*10^{-5}$M

By using the equation above i find that the concentration of H+ is 2.67E-10

But if I use a different method as follows:

-log[OH]=pOH so by substitution, -log[$3.57*10^{-5}$]

then that gives me 4.425.......

then i subtract 14 from that number to get 9.574......

which that is pH

then i set 9.574=-log[H+] so then you "e" everything which the answer comes to be 6.95E-5

why is this wrong?

• Do you know the difference between $$9.574 = -\log x$$ and $$9.574 = -\ln x?$$ – orthocresol Jan 8 '17 at 8:52
• Oh no wonder, i get it now – John Rawls Jan 8 '17 at 8:54