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Topic: PH of a buffer made by mixing two solutions  (Read 9779 times)

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Offline claire18

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PH of a buffer made by mixing two solutions
« on: March 22, 2010, 04:43:23 PM »
Hi, could anyone help with this. I know its properly simple but I cant get my head round it. Thanks

Find the PH of a buffer made by mixing 100cm3 of 0.100 molar benzoic acid (PKa = 4.202) and 200cm3 of 0.050 molar sodium benzoate. 

Offline Borek

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Re: PH of a buffer made by mixing two solutions
« Reply #1 on: March 22, 2010, 04:47:15 PM »
What equation is used for buffer pH calculation?
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Offline claire18

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Re: PH of a buffer made by mixing two solutions
« Reply #2 on: March 23, 2010, 08:45:28 PM »
[H+] = Ka [HA]/[A-]

or

PH = PKa + log ([A-]/[HA])

Offline sjb

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Re: PH of a buffer made by mixing two solutions
« Reply #3 on: March 24, 2010, 03:00:09 AM »
So what, for instance are [A] and [HA] here?

Offline claire18

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Re: PH of a buffer made by mixing two solutions
« Reply #4 on: March 24, 2010, 09:02:00 AM »
I think 0.100 and 0.050 but i really dont know to be honest  :-[

Offline Borek

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Re: PH of a buffer made by mixing two solutions
« Reply #5 on: March 24, 2010, 09:37:50 AM »
What is A-, what is HA, how are they related to the substances given in question?
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Offline claire18

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Re: PH of a buffer made by mixing two solutions
« Reply #6 on: March 24, 2010, 12:32:35 PM »
[A−] is the concentration of the conjugate base and [HA] is the concentration of the acid.

What i don't get is which is which, sorry

Offline Borek

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Re: PH of a buffer made by mixing two solutions
« Reply #7 on: March 24, 2010, 03:05:18 PM »
Try to apply definitions.

Note, that even looking at symbols (HA & A-) should give you a strong hint...
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Offline claire18

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Re: PH of a buffer made by mixing two solutions
« Reply #8 on: March 24, 2010, 08:23:15 PM »
A- is 0.050 and HA is 0.100

i think!

Offline Matias Ekstrand

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Re: PH of a buffer made by mixing two solutions
« Reply #9 on: March 25, 2010, 01:17:04 AM »
You're right about which chemical is the acid and which is the conjugate base, but make sure to take into account that you mix two solutions and change the volume (which changes the concentrations).
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Offline claire18

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Re: PH of a buffer made by mixing two solutions
« Reply #10 on: March 25, 2010, 02:44:39 PM »
So [HA] is 10 and [-A] is 10, is this right and what i do next?

Sorry about this

Offline sjb

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Re: PH of a buffer made by mixing two solutions
« Reply #11 on: March 25, 2010, 04:06:17 PM »
No, [HA] was 0.1 molar (the benzoic acid), making 0.01 moles in total. What is the total volume of solution now?

Offline Borek

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Re: PH of a buffer made by mixing two solutions
« Reply #12 on: March 25, 2010, 05:58:06 PM »
So [HA] is 10 and [-A] is 10, is this right and what i do next?

You are still just guessing till someone tells you you hit the target. You won't learn anything this way.

What are definitions of acid and conjugate base?
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Offline claire18

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Re: PH of a buffer made by mixing two solutions
« Reply #13 on: April 21, 2010, 01:09:50 PM »
PH = PKa + log [acid/salt]
PH = 4.202 + log [0.100/0.050]
PH = 4.50

is this right?

Offline Borek

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Re: PH of a buffer made by mixing two solutions
« Reply #14 on: April 21, 2010, 01:50:15 PM »
No.
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