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Offline tjpj77

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Find pH if...
« on: September 05, 2012, 04:40:08 AM »
Find pH if 3 mL 0.1M HCl is added to 97 mL of pure H2O at pH 7.0.

(3mL/100mL)(0.1 mol HCl/1L) = 3 x 10^-3 mol HCl/L
3 - log 3 = 2.5
pH = 2.5
right?

Offline Dan

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Re: Find pH if...
« Reply #1 on: September 05, 2012, 05:17:00 AM »
Find pH if 3 mL 0.1M HCl is added to 97 mL of pure H2O at pH 7.0.

(3mL/100mL)(0.1 mol HCl/1L) = 3 x 10^-3 mol HCl/L

OK

Quote
3 - log 3 = 2.5

What are you doing here? Where does this come from?
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Offline tjpj77

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Re: Find pH if...
« Reply #2 on: September 05, 2012, 05:23:02 AM »
after you have the M of a strong acid or base, you can just do the quick math via that trick: bring down and cross over the power and then subtract from that the log of what was the first number.

Offline Borek

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Re: Find pH if...
« Reply #3 on: September 05, 2012, 05:58:59 AM »
Hardly a trick, just using log properties:

log(3×10-3) = log(3) + log(10-3) = ~0.5 - 3
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Offline Dan

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Re: Find pH if...
« Reply #4 on: September 05, 2012, 06:25:39 AM »
Ah OK. I agree with 2.5, I was just wondering where that calculation came from. I've never bothered to expand the log before calculating for questions like this.
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Offline Borek

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Re: Find pH if...
« Reply #5 on: September 05, 2012, 06:31:46 AM »
For me it was obvious.

Most likely that shows the difference between me and the calculator generation  :P
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Offline Dan

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Re: Find pH if...
« Reply #6 on: September 05, 2012, 06:35:20 AM »
I was thinking similarly  ;)
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