Converting pOH into [OH-] question

[KristenD77]

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Hello can someone please help me with this problem. I'm having the hardest time figuring it out.

If the pH is 4.9 what is the [OH-]

So I know that the pOH is -9 so the [OH-] would be 10^ -9 but the answer is 7.9 x 10 ^ -10

I really can't figure out how to get that number without a calculator
Help would be GREATLY appreciated!

 

[KristenD77]

POH = 14 - 4.9 = 9.1
10^-9.1 = 7.9*10^-10

You're getting 10^-9 because when you approximate you get 1*10^-9.

9.1 will give you slightly less than 1 (0.79), and you have to give your answer in proper scientific notation, which is 7.9*10^-10.

Thank you so much!