log calculations

[mintz123]

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how does anyone figure out pOH= -log 3.5x10-2, pOH =1.5ish; how do you solve log problems quickly on a timed test w/o calculators? is there a startegy i'm not aware of whether its log of 3.5x10-5 or 3.5x10-2?😕😕

 

[littlebittyjk]

this has been bugging the heck outta me too! I can't find anything online that explains it either. Anyone out there able to explain it bc Orgoman sure loves to use those!

 

[Andrew324]

well I also have troubles but ill explain it as I have come to understand it, hope it helps

ok 1x10^-2 when you do a log of that its 2
-Log 1x10^-3 = 3
-Log 1x10^-4 = 4
-Log 1x10^-5 = 5

Anyway when you throw in some other numbers instead of 1 it changes it up
-Log 1x10^-3 = 3
-Log 5x10^-3 = 2.5 (roughly actually =2.3)
-Log 9x10^-3 = 2 (its just over 2, really 2.04)

Does this help at all?

 

[Albuterol]

there was a quick little tip in the kaplan blue book gchem section p289.
here it goes:

when you have a log with a base of 1 the answer is the inverse of your exponent.
for example log 1 X 10e-4 = 4

things can get a little trickier when you have base that is not 1 but w/this trick it should be rather quick and easy;

for example we have -log 3.5x10-2. what you want to do is take the exponent and make it positive so now we have 2 and then subtract log 3.5 from the positive exponent. our equation now looks like 2-log 3.5 = ?....since we know log 10= 1 we can approximate that log 3.5 is less than 1 and your answer will be between 2 and 1. and that is the case pOH = 1.5. Hope this helps.

 

[fsudental]

how does anyone figure out pOH= -log 3.5x10-2, pOH =1.5ish; how do you solve log problems quickly on a timed test w/o calculators? is there a startegy i'm not aware of whether its log of 3.5x10-5 or 3.5x10-2?


ITS Easy- What ever number is on the exponent, your answer would be between that number and - 1.
Example
pOH= -log4.2x 10-7

Answer= Between 6 and 7

I HOPE THIS HELPS

 

[mintz123]

ahhhh thank you!! now i just have to go back and redo every log problem!! 😱😱