March 29, 2024, 06:52:18 AM
Forum Rules: Read This Before Posting


Topic: PH of a buffer made by mixing two solutions  (Read 11313 times)

0 Members and 1 Guest are viewing this topic.

Offline claire18

  • Regular Member
  • ***
  • Posts: 15
  • Mole Snacks: +0/-0
  • Gender: Female
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

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27634
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
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?
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline claire18

  • Regular Member
  • ***
  • Posts: 15
  • Mole Snacks: +0/-0
  • Gender: Female
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

  • Global Moderator
  • Sr. Member
  • ***
  • Posts: 3652
  • Mole Snacks: +222/-42
  • Gender: Male
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

  • Regular Member
  • ***
  • Posts: 15
  • Mole Snacks: +0/-0
  • Gender: Female
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

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27634
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
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?
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline claire18

  • Regular Member
  • ***
  • Posts: 15
  • Mole Snacks: +0/-0
  • Gender: Female
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

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27634
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
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...
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline claire18

  • Regular Member
  • ***
  • Posts: 15
  • Mole Snacks: +0/-0
  • Gender: Female
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

  • Regular Member
  • ***
  • Posts: 50
  • Mole Snacks: +2/-0
  • Gender: Male
    • ImproveScience.com
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).
ImproveScience.com - A free source of chemistry information.

Offline claire18

  • Regular Member
  • ***
  • Posts: 15
  • Mole Snacks: +0/-0
  • Gender: Female
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

  • Global Moderator
  • Sr. Member
  • ***
  • Posts: 3652
  • Mole Snacks: +222/-42
  • Gender: Male
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

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27634
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
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?
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline claire18

  • Regular Member
  • ***
  • Posts: 15
  • Mole Snacks: +0/-0
  • Gender: Female
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

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27634
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: PH of a buffer made by mixing two solutions
« Reply #14 on: April 21, 2010, 01:50:15 PM »
No.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Sponsored Links