Chemical Forums

Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: mpire344 on September 03, 2009, 09:49:08 PM

Title: Determining proper volumes of acid and base for buffer solution.
Post by: mpire344 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.

Title: Re: Determining proper volumes of acid and base for buffer solution.
Post by: Borek on September 04, 2009, 03:06:47 AM

Show your calculations, no problems with getting correct pH with these solutions.

Title: Re: Determining proper volumes of acid and base for buffer solution.
Post by: mpire344 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.

Title: Re: Determining proper volumes of acid and base for buffer solution.
Post by: Borek 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.

Title: Re: Determining proper volumes of acid and base for buffer solution.
Post by: mpire344 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.

Title: Re: Determining proper volumes of acid and base for buffer solution.
Post by: mpire344 on September 04, 2009, 07:47:57 AM

Snacks for all. Huzzah!