ph calculations precision

[hope_to_match]

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how precise do we have to be with calculations on test day? this is one of chad's quiz question. pOH of 5.1 and 4.9 are pretty close which can lead one to the wrong answer (for ph in this case 8.9 or 9.1) if not predicted close enough. Thank you 🙂

 

[Illfavor]

Not really an issue in this case if you understand the math. The pOH has to be greater than 5 therefore the pH has to be less than 9. I imagine they could make it this close just to test the concept.

 

[hope_to_match]

Not really an issue in this case if you understand the math. The pOH has to be greater than 5 therefore the pH has to be less than 9. I imagine they could make it this close just to test the concept.

ok well then I am not fully understanding the math behind it. I understand why pOH will be less than 6 but I am not sure why it will be greater than 5..can you explain, please ?

 

[PointEstimate]

I don't remember having to be that exact....

This helped me out a ton when I was practicing:
http://forums.studentdoctor.net/showpost.php?p=8100215&postcount=2

if [OH] = 2.0 x 10^-4, then u can say pOH = 4 - log 2 = 3.7
you should try to memorize some simple logs, at least 1-5:
log 1 = 0
log 2 = 0.3
log 3 = 0.5
log 4 = 0.6
log 5 = 0.7
log 6 = 0.8
log 7 = 0.85
log 8 = 0.90
log 9 = 0.95
log 10 = 1

 

[Illfavor]

ok well then I am not fully understanding the math behind it. I understand why pOH will be less than 6 but I am not sure why it will be greater than 5..can you explain, please ?

1x10^-6 = pOH of 6
1x10^-5 = pOH of 5
9x10^-6 = pOH of 5.1

We have a pH of ~9x10^-6, which is nearly 10x10^-6, which = 1x10^-5. But since we have a lower OH concetration than 1x10^-5, our pOH has to be a larger number than 5 (remembering that as we increase the [OH] the pOH gets smaller). So, we know it has to be between 5-6, or a pH between 8-9.

 

[hope_to_match]

1x10^-6 = pOH of 6
1x10^-5 = pOH of 5
9x10^-6 = pOH of 5.1

We have a pH of ~9x10^-6, which is nearly 10x10^-6, which = 1x10^-5. But since we have a lower OH concetration than 1x10^-5, our pOH has to be a larger number than 5 (remembering that as we increase the [OH] the pOH gets smaller). So, we know it has to be between 5-6, or a pH between 8-9.

alright thanks! Now I remember Chad talking about these calculations..hehe i think i got the concepts down but just brushing up on small things now. Thanks again