March 29, 2024, 01:59:17 AM
Forum Rules: Read This Before Posting


Topic: pH change in a buffer with addition of HCl  (Read 5042 times)

0 Members and 1 Guest are viewing this topic.

Offline Simons2

  • Regular Member
  • ***
  • Posts: 19
  • Mole Snacks: +0/-0
pH change in a buffer with addition of HCl
« on: September 17, 2017, 12:26:35 PM »
I have to predict the change in pH when 0.50ml of 1M HCl is added to 40mL of .10M buffer at pH 6. The buffer was made with Na2HPO4 and NaH2PO4.

I did this successfully for a pH of 7.5; pH=pKa+log[b/a]; 7.5=7.2+log[b/a]; b=2a. Now I kept everything in milmols instead of changing it to moles so a+b=4mmols a=1.33 b=2.7.
HPO4 + H+ ==> H2PO4
2.7.. 0 ... 1.3 -The addition of the .5mL of HCl ends up with the final being: 2.2mmol of HPO4 and 1.8mmol of H2PO4. Plugging those numbers into the HH equation: 7.2+log[2.2/1.8]=7.29 so the change in pH is 7.5-7.29 = .21. I understood how to do all this.

Now if I were to do this with pH 6, I would end up with a negative concentration for HPO4 and you can't plug a negative number into a log. Following the steps above; b=.0631a; a+b=4mmol a=3.76 b=.237. Add the HCl and you get a negative for b: (.237-.5=-.263). I understand when you get a number that is negative you have excess HCl but I can't work with a negative number in pH=pKa+log[b/a]. I have no idea what I'm doing wrong. I also have to do this for 40mL of .01M buffer with the addition of .5mL of 1M HCl as well, and all those numbers will also be negative. Do I just take the absolute value of it or is my math wrong?

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27633
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: pH change in a buffer with addition of HCl
« Reply #1 on: September 17, 2017, 12:52:25 PM »
You are moving into another buffer territory - H3PO4/H2PO4-.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline Simons2

  • Regular Member
  • ***
  • Posts: 19
  • Mole Snacks: +0/-0
Re: pH change in a buffer with addition of HCl
« Reply #2 on: September 17, 2017, 01:00:44 PM »
You are moving into another buffer territory - H3PO4/H2PO4-.

I don't understand how that helps when solving for pH change. I understand it hypothetically because of all the excess HCl in solution, but mathematically I don't see the relevance/how to use it for solving the problem.   

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27633
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: pH change in a buffer with addition of HCl
« Reply #3 on: September 17, 2017, 04:08:21 PM »
Follow the stoichiometry - once all HPO4- gets protonated into H2PO4- excess HCl (the one giving you negative concentrations) will start to protonate H2PO4-  into H3PO4. You need to calculate the concentrations of H2PO4- and H3PO4 and use pKa1 to calculate pH (this approach won't give an exact answer, but finding a better one is probably beyond the scope of the exercise).
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline Simons2

  • Regular Member
  • ***
  • Posts: 19
  • Mole Snacks: +0/-0
Re: pH change in a buffer with addition of HCl
« Reply #4 on: September 17, 2017, 07:13:34 PM »
I've figured it out, but thank you! I do have a different question tho;

I'm asked to calculate the pH using phosphate correction factors when 0.50ml of 1M HCl is added to 40mL of .10M buffer at expected pH 6 and pH 6.5. So same numbers as above. I know the correction factors: Correction factors for .10M: HPO4-0.45 H2PO4-0.74 as well as the equation pH=pKa +log[b/a]+log[cf of b/cf of a]. I've been getting very messed up answers that don't come close to what I've measured in the lab. When I calculated it for pH's 7, 7.5,8 they all came close to my measured pH. pH 6 and 6.5 didn't. I'm unsure what to do.

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27633
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: pH change in a buffer with addition of HCl
« Reply #5 on: September 18, 2017, 01:24:24 PM »
Show your work, hard to comment on something we can't see.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Sponsored Links