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### Topic: Determining proper volumes of acid and base for buffer solution.  (Read 3455 times)

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#### mpire344

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##### Determining proper volumes of acid and base for buffer solution.
« on: September 03, 2009, 09:49:08 PM »
I need to determine the volumes of 0.10 M NaH2PO4 and 0.10 M Na2HPO4 that need to be mixed
together to produce 100 mL of a buffer at pH 7.0. The second pKa of H3PO4 is 7.2.

The problem I seem to run into is that the molarities of the two components are too weak to create that buffer in a combined total 100 mL of each of the compounds.

Any help would be appreciated.

#### Borek

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##### Re: Determining proper volumes of acid and base for buffer solution.
« Reply #1 on: September 04, 2009, 03:06:47 AM »
Show your calculations, no problems with getting correct pH with these solutions.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

#### mpire344

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##### Re: Determining proper volumes of acid and base for buffer solution.
« Reply #2 on: September 04, 2009, 07:28:28 AM »
7= 7.2 + log A/HA

A/HA= 0.630

A= 0.630HA

Let X = amount of H2PO4 used (mL)

0.630X + X = 100 Ml (volume of buffer needed for problem)

x= 61.31 mL H2PO4

100ml-61.31= 38.69 mL HPO4

Is this method methodologically sound? When I calculate moles of each using this method and insert the corresponding answers into the Henderson-Hasselbach equation, I get a ph thats greater than 7.

#### Borek

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##### Re: Determining proper volumes of acid and base for buffer solution.
« Reply #3 on: September 04, 2009, 07:44:34 AM »
Looks perfect to me.

Perhaps you do something wrong when you are trying to calculate pH afterwards.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

#### mpire344

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##### Re: Determining proper volumes of acid and base for buffer solution.
« Reply #4 on: September 04, 2009, 07:47:05 AM »
Borek, I realize what I was doing wrong.

When I was calculating base/acid ratio for Henderson-Hasselbach, I was subtracting A- moles from HA moles because I thought one neutralized the other.

Anyways thanks for your help.

#### mpire344

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##### Re: Determining proper volumes of acid and base for buffer solution.
« Reply #5 on: September 04, 2009, 07:47:57 AM »
Snacks for all. Huzzah!