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### Topic: Calculating pH problems  (Read 1803 times)

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#### ohaychminus

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##### Calculating pH problems
« on: September 16, 2012, 12:31:45 PM »
Hey, all. I posted here yesterday asking a similar question. This time, though, I'm having problems calculating the pH of a solution. Here's the question:

19.21 mL of 0.1679 M aqueous HCl is added to 20.68 mL of 0.2752 M aqueous NaOH at 25°C. Assuming the volumes are additive, what is pH of the resulting solution?

I started off by calculating the number of moles of HCl and NaOH (0.003225 and 0.005091, respectively).

Shouldn't I be able to calculate [OH-] like so: (0.005091 - 0.003225)/0.03989 = 0.04677

Then, since I have [OH-], I could do this: -log(0.04677) to obtain pOH, and then 14 - pOH to get pH.

The answer I get from this isn't being accepted.

#### Hunter2

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##### Re: Calculating pH problems
« Reply #1 on: September 16, 2012, 12:42:37 PM »
Calculate the moles of HCl and NaOH
then do the difference of it. What is left OH- or H+. The moles what you obtained calculate to the new volume.

#### ohaychminus

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##### Re: Calculating pH problems
« Reply #2 on: September 16, 2012, 12:59:40 PM »
Isn't that what I did, though?

#### Borek

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##### Re: Calculating pH problems
« Reply #3 on: September 16, 2012, 01:33:36 PM »
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

#### ohaychminus

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##### Re: Calculating pH problems
« Reply #4 on: September 16, 2012, 02:01:22 PM »
Ah, I see I made a mistake in calculating the moles of NaOH.

Is my methodology for calculating pH this way correct, though?

• Mr. pH