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### Topic: Calculate log() and 10^x without a calculator  (Read 788 times)

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#### Judy

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##### Calculate log() and 10^x without a calculator
« on: January 16, 2022, 11:32:28 PM »
Hello,

The following is my calculation for the first attachment:
1.6*10^-14 = 10^(-0.85) * [Fe2+]
How could I calculate 10^(-0.85) without a calculator?

The following is my calculation for the second attachment:
pH = -log(1.7*10^-5) + log(0.005/0.01)
How could I calculate the two logs without a calculator?

Thank you!

#### Borek

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##### Re: Calculate log() and 10^x without a calculator
« Reply #1 on: January 17, 2022, 02:59:58 AM »
In general that's what old fashioned paper log tables are for.
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#### Judy

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##### Re: Calculate log() and 10^x without a calculator
« Reply #2 on: January 17, 2022, 11:31:19 AM »
I see, thank you!

#### mjc123

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##### Re: Calculate log() and 10^x without a calculator
« Reply #3 on: January 17, 2022, 04:42:04 PM »
Your first calculation is wrong. What is the expression for Ksp of Fe(OH)2?

#### Corribus

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##### Re: Calculate log() and 10^x without a calculator
« Reply #4 on: January 18, 2022, 11:22:20 AM »
Neglecting the chemistry, you can go a long way toward simplifying equations with relationships log(ab)=log(a)+log(b) and log(a/b)=log(a)-log(b)
Hard to compute actual numbers (as Borek mentioned, log tables) but frequently you can estimate them and choose a correct answer in multiple-choice format.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman