Experiment8.BuffersI...
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Experiment 8. Buffers I 19 March 2020 Weak acid + conjugate weak base = buffer. Photo: Science in Hydroponics
Transcript of Experiment8.BuffersI...
Experiment 8. Buffers I 19 March 2020
Weak acid + conjugate weak base = buffer.
Photo: Science in Hydroponics
Objec&ve: To prepare a buffer of a specific assigned pH from a weak acid and its conjugate weak base.
Overview: 1. Review making buffers and the Henderson
Hasselbalch Buffer Equa&on 2. Choosing the best weak acid + conjugate base pair 3. The Math 4. Making the buffer 5. Procedure: What we do today 6. Your lab report
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This week and next week we will be making buffers! This could be
fun…
…and educaEonal.
1. Review making buffers and the Henderson Hasselbalch Buffer Equa&on
There are three recipes for making buffers. This week we will be using the
first recipe.
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And next week we will use either Recipe 2 or 3.
The 3 recipes for making a buffer: 1. Mix a weak acid with its conjugate
weak base 2. React a weak acid with OH-‐ as the
limi&ng reagent 3. React a weak base with H3O+ as the
limi&ng reagent
What could possibly go wrong?
pH = pKa + log( ) nwb nwa
So looking at the Henderson, we see that the pH depends on the
number of moles of weak acid and moles of weak base… and the pKa, of
course.
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1. Review making buffers and the Henderson Hasselbalch Buffer Equa&on
Usually there is more of one than the other in order
to get the desired pH.
NoEce that if the moles of weak base equals the moles of weak acid, the log term equals zero… and pH = pKa.
Info for IntroducFon
2. Choosing the best weak acid + conjugate base pair
pH = pKa + log( ) nwb nwa
HA pKa = 4.75 HB pKa = 5.15 HC pKa = 6.05 HD pKa = 7.45
Then we can mix similar moles of weak acid and weak base.
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So the strategy is to pick an acid with a pKa similar to
the desired pH.
pH = pKa + log( ) nwb nwa
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2. Choosing the best weak acid + conjugate base pair
So if the assigned pH was 6.00, for example, the best acid for the job would be HC and its conjugate base C-‐.
HA pKa = 4.75 HB pKa = 5.15 HC pKa = 6.05 HD pKa = 7.45 And now we do
a liVle math.
3. The math
nwb nwa nC-‐ nHC
= 0.8913 nC-‐ nHC
ConEnuing on with this pH 6.00 example, we need to make a
soluEon in which the mole raEo of weak base to weak acid is 0.891
First solve for log(wb/wa). It’s -‐0.05. And then take the inverse log10.
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-‐0.05 = log( ) nC-‐ nHC
So with log numbers, every digit to the right of the decimal is significant. The number -‐0.05 has two significant figures. I know! Crazy. For sig figs, we get 0.89, but we should use
0.8913 through to the end of the calculaEons.
6.00 = 6.05 + log( )
pH = pKa + log( )
Try this calculaEon yourself. See if you
get 0.8913.
= 0.891 nC-‐ nHC
The instrucEons said to make 50.0 mL of 0.100 M buffer.
nC-‐ + nHC = 0.0050 moles
From n = MV, that means we want to make 0.0050 total moles of buffer (moles weak
acid + moles of conjugate base)
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3. The math
Wait. Two equaEons and two
variables!
Info for IntroducFon
= 0.891 nC-‐ nHC
We solve for nHC using a liVle trick called math.
nC-‐ + nHC = 0.0050 moles
0.891nHC + nHC = 0.0050 moles 1.891nHC = 0.0050 moles total nHC = 0.00264 moles HC
nC-‐ = 0.891nHC
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3. The math
Two equaEons …two variables…
Hmmm...
Then subtract. Boom. Done.
nC-‐ + nHC = 0.0050 moles
nHC = 0.00264 moles HC nC-‐ = 0.00236 moles C-‐
nHC = 0.00264 moles HC
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3. The math
So this is our plan – to measure out this many moles of HC and C-‐. Dissolve them together in water. Voila!
ntotal = 0.00500 moles
Of course each pair of us will have a different
assigned pH, so let’s start with picking the
appropriate weak acid – the one with the pKa that matches our assigned pH.
Easy peasy. Convert from moles to mass and mass out the weak acid and weak base and dissolve them in about 50
mL water!
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4. Making the buffer
To review the process: 1. Use Henderson to solve for the raEo nwb/nwa. 2. Given nwb + nwa = 0.0050 mol, solve for nwb and nwa. 3. Convert the moles to
masses and 4. Dissolve in deionized water!
Nothing going wrong so far…
Weak Acid pKa MM Weak Base MM NaHSO4 1.990 120.06 g/mol Na2SO4 142.04 g/mol (sodium hydrogen sulfate (sodium sulfate) or sodium bisulfate)
NaH2PO4 7.198 119.98 g/mol Na2HPO4.7 H2O 268.07 g/mol
(sodium dihydrogen phosphate (sodium hydrogen phosphate or sodium phosphate monobasic) or sodium phosphate dibasic)
NaHCO3 10.320 84.01 g/mol Na2CO3 105.99 g/mol (sodium hydrogen carbonate (sodium sulfate) or sodium bicarbonate)
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4. Making the buffer
These are our choices today for weak acids and conjugate bases.
Start by picking the best one for your assigned pH.
B. Use Henderson to figure out mole raEo and moles of each C. Convert moles
to masses.
D. Make the buffer – the exact amount of water doesn’t maVer, but should be around 50 mL.
A. Pick the best wa + wb from previous slide for your
assigned buffer.
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4. Making the buffer
Info for IntroducFon
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4. Making the buffer
The buffer probably won’t work out like the math – it’ll be close. We can make adjustments quite easily, however.
If the pH is too high, we can add drops of 1 M
HCl(aq) unEl it’s perfect.
Hmmm… What should I do if the pH is too low?
See? Something went wrong.
Info for IntroducFon
TWEEK THE pH ‘TIL IT’S PERFECT!
DONE!
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5. Procedure: What we do today
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Today we follow the procedure as described in the lab manual starEng
on page 53.
Almost seems a liVle too easy…
Yup. Nothing to add.
Then come back here for the calculaEons starEng on Slide 5.
TWEEK THE pH ‘TIL IT’S PERFECT!
DONE!
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5. Procedure: What we do today
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Stuff to do today….
① Wear your safety glasses. Dress for a mess.
② Take Eme wriEng an introducEon in your own words before lab.
③ Record observaEons and details as carefully as possible. ④ Once you are ready to make your buffer, use the pocket
balances at your staEon. They are good to the hundredths place.
⑤ When your buffer is ready, ask your TA to check it for accuracy. She/he will iniEal the cover sheet if it’s ok. Then clean up and complete your on-‐line submission!
⑥ The cover sheet summarizes everything that you need to include with your report.
⑦ Store the pH probes in the 0.01 M HCl provided at the middle table common area. Goldfish, prefer clean fishbowl water.
17 Chem Lab with the S.ck People and Bird was created and produced by Dr. Bruce
MaVson, Creighton Chemistry. Enjoy it and share it if you wish.
6. Your Lab Report
① First, the cover page with TA ini&als. ② Next, the trimmed copy pages from your lab
notebook stapled together. ③ Staple together – nothing to print today. ④ On-‐line results due at the end of class today. Late
submissions are not graded – see the syllabus. ⑤ Turn in lab report today or before the start of
class tomorrow. Late labs may not be graded – see the syllabus.
S&ck people inspired by xkcd cartoons by Randall Munroe
(www.xkcd.com)
Hope your St PaVy’s is enjoyable. Umm, what
was I thinking? This is
embarrassing…
