December 26, 2024, 10:12:27 AM
Forum Rules: Read This Before Posting


Topic: Calculate buffert pH  (Read 4238 times)

0 Members and 1 Guest are viewing this topic.

Offline hahahanna

  • Regular Member
  • ***
  • Posts: 23
  • Mole Snacks: +0/-0
Calculate buffert pH
« on: August 28, 2009, 12:06:30 PM »
Hi!
I have 2 questions about making buffers:

Im going to calculate a buffert with benzoic acid and NaOH.

1. first i want to calculate the buffer pH, and i have dine like this:

Vtot= 200*10-3ml
mbenzoic=1,32 g
Mwbenzoic=122g/mol
VNaOH= 24*10-3 ml
CNaOH= 1 M

I have calculate the concentration for benzoic acid and NaOH = Cbenzoic acid=0,05 M and NaOH=0,12M

I will use Henderson–Hasselbalch equation..
And the pKa for benzoic acid is 4,19.. So with the equation i will get the pH at the buffer: 6,59.

Have i thinking right now?

2. My other question is that i want to calculate pH in the same buffer above, only that i will take just 20 ml from it and then establish 1ml, 1M NaOH. What do i calculate with if i want to do that. and how? We can say that i want to take out 20 ml of my stocksolution and establish 1ml 1M NaOH to that.
Is it the concentration? But i which one do i use? The concentration for benzoic acid or NaOH???

please, i really need help.
And i have no book to look in.


thanks!

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27887
  • Mole Snacks: +1816/-412
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Calculate buffert pH
« Reply #1 on: August 28, 2009, 01:51:43 PM »
Please elaborate - you are dropping some numbers but you are not clearly stating what the question is.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline hahahanna

  • Regular Member
  • ***
  • Posts: 23
  • Mole Snacks: +0/-0
Re: Calculate buffert pH
« Reply #2 on: August 28, 2009, 02:26:12 PM »
My question is:
I want to do a buffer with benzoic acid and NaOH, and I will call this buffer when its finish for “Solution A”.
I want to have 200 ml of the buffer, and I have 1,32 g benzoic acid and 24 ml of 1,00 M NaOH.

C6H5COOH + OH-  C6H5COOH + H2O

1. first i want to calculate the buffer pH, and i have done like this:

Vtot= 200*10-3ml
mbenzoic=1,32 g
Mwbenzoic=122g/mol
nbenzoic= 0,01 mol

VNaOH= 24*10-3 ml
CNaOH= 1 M
nNaOH= 0,024 mol
Then I calculated the concentration in 200 ml:
CNaOH= n/V => 0,024/200*10-3 = 0,12 M
Cbenzoic= n/V => 0,01/200*10-3= 0,05 M
Now I will use Henderson–Hasselbalch equation..
And the pKa for benzoic acid is 4,19.. So with the Henderson–Hasselbalch equation i will get the pH at the buffer: 6,59.

Have i thinking right now?

2. My other question is that i want to calculate pH if I take 20 ml “Solution A” (the solution above) and establish 1ml, 1M NaOH. How do I calculate this pH?
Is it the concentration for benzoic acid that I use for the calculation?


Sponsored Links