pH rule question

[diene]

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Does the power of rule that each power of 10 represents 1 pH unit only apply to to the hydronium concentration? What do you do if given the hydroxide concentration? Can someone explain this question?

What is the pH of a solution where the hydroxide concentration is 10^6 times greater than the hydronium concentration?
A. 8
B. 6
C. 10
D. 13.
The answer is C.

 

[mario0923]

Does the power of rule that each power of 10 represents 1 pH unit only apply to to the hydronium question? What do you do if given the hydroxide concentration? Can someone explain this question?

What is the pH of a solution where the hydroxide concentration is 10^6 times greater than the hydronium concentration?
A. 8
B. 6
C. 10
D. 13.
The answer is C.

since [H+][OH-]=1e-14

and we know [H+]=[OH-]x1e6

than [H+]=1e-10
[OH-]=1e-4
do you see how 1e-4 is 1e6 times greater than 1e-10 and (1e-4)(1e-10)=1e-14?

so now we got [H+]=1e-10
pH=10 or [OH-]=1e-4 pOH=4 so pH=14-pOH=14-4=10

just remember pH+pOH=14 and [H+][OH-]=1e-14

 

[diene]

Oh I see where I went wrong..thanks!

 

[votruong]

There is another way to do this:

[H+][OH-] = 10^-14 (1)
[OH-] = 10^6[H+] (2)
sub (2) to (1) we have [H+]*10^6[H+] = 10^-14
[H+][H+] = 10^-14/10^6
[H+]^2 = 10^-20
[H+] = 10^-10
pH = -log10^-10 = 10