Chad's pH and pOH calculations

[valkyrie08]

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Can someone please help me to understand Chad's shortcut in this topic?

Question: If [H+] = 6.4x10-8M, what is the pH of the solution?

Solution: pH = -log [H+]
pH = -log (6.4x10-8) which should be between 7 and 8. As [H+] increases, pH decreases (they’re inversely related). Since the -log of 1x10-8 = 8, then the -log of a slightly higher [H+] will yield a slightly lower pH.

But this still leaves us with two possible correct answer choices (7.2 and 7.8) and so you’ll have to remember not only the [H+] values when the pH is an integer but also for the halves (.5) too. The half pH occurs when [H+] = 3.16x10-whatever as demonstrated below:

[H+] = 1x10-7M pH = 7
[H+] = 3.16x10-8M pH = 7.5
[H+] = 1x10-8M pH = 8

Since 6.4x10-8 is between 3.16x10-8 and 1x10-7, then the corresponding pH will be between 7.5 and 7 making 7.2 the correct answer in this case.



Why is 6.4x10-8 between those two numbers? Or is there an easier way to solve this? Thank you!

 

[syeh3]

I've only just figured out to do these even tho my exam is this Saturday ~ basically look at the exponent which here I got -8 so pH would definitely fall between 7 and 8

 

[syeh3]

And think of maybe what is half the number u have?

 

[djwon1]

http://forums.studentdoctor.net/threads/log-2-10-4-mental-math.884665/#post-15284958

 

[valkyrie08]

Yeah but he goes into saying how it needs to be .5 so why is ph 8 between 7 and 7.5? That doesn't make sense to me

 

[valkyrie08]

Thanks for the replies and the link! Helped a lot!

Good luck on your exam syeh3!

 

[djwon1]

Honestly it doesnt hurt to know wheter it's over .5 or under .5 but i am pretty sure you won't see this on the exam. Focus on concepts more !