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### Topic: Find pH if...  (Read 2036 times)

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#### 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?

#### 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?
My research: Google Scholar and Researchgate

#### 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.

#### 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
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

#### 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.
My research: Google Scholar and Researchgate

#### 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
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

#### Dan

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##### Re: Find pH if...
« Reply #6 on: September 05, 2012, 06:35:20 AM »
I was thinking similarly
My research: Google Scholar and Researchgate