Experiment9 22*March*2018*...
Transcript of Experiment9 22*March*2018*...
Experiment 9 22 March 2018 Designer Buffers
Welcome to the World of Designer Buffers – where
specialty buffers for research and medical applica;ons are sold
for big money.
h"p://cellgro.com/products/microbiology-‐and-‐molecular-‐biology/buffers-‐buffers/phosphate-‐buffered-‐saline-‐1x.html
I’m dressed for big money!
Objec>ve: To prepare a buffer of a specific assigned pH using Recipe 2 or 3.
Last week we used Recipe 1 to make a buffer of assigned pH. This week we will use either Recipe 2 or
3 to make a specific buffer.
As it turns out, Recipes 2 and 3 are far more commonly used than Recipe 1
when making buffers.
When do we get to the big money part? 2
Overview: 1. Review making buffers and the Henderson Hasselbalch
Buffer Equa>on 2. Choosing the best weak acid + conjugate base pair 3. The math for Recipe 2: Star>ng with weak acid and
adding hydroxide to make the buffer 4. The math for Recipe 3: wb + sa 5. What we are doing in lab today 6. Your lab report
1. Review making buffers and the Henderson Hasselbalch Buffer Equa>on
There are three recipes for making buffers – as you
recall. This week we will be using either Recipe 2 or Recipe 3.
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We can’t use Recipe 1 unless we have both the
weak acid and its conjugate base.
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
So how does this work, anyway?
pH = pKa + log( ) nwb nwa
So here is Henderson again – the equa;on we use whenever we have a
buffer situa;on.
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1. Review making buffers and the Henderson Hasselbalch Buffer Equa>on
Go Moles! n = MV
With Recipes 2 and 3, we must work in moles. With Recipe 1, we could use moles or molarity, even
though working in moles is always beTer.
Info for IntroducFon
5 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 Even if we end up
star;ng with a weak base (Recipe 3), we
shop for the weak acid with a pKa closest to
the desired pH. 5
So the strategy is to pick a weak acid with a pKa similar to the desired pH
Last week, in the example we worked in the pre-‐lab material, we picked HC because we were
assigned a pH = 6.00.
6 Info for
IntroducFon
2. Choosing the best weak acid + conjugate base pair.
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But this week, we will all be working with a buffer that consists of H2PO4
-‐(aq) as the weak acid and HPO4
2-‐(aq) as the conjugate weak base.
The weak acid H2PO4-‐(aq) has a pKa
= 7.198. This number has 3 significant figures – the 198 part. The “7” represents an exponent.
The corresponding Ka is 6.38 x 10-‐8.
Some groups will start with the weak acid, H2PO4
-‐, in the form of NaH2PO4(s), MM = 119.98 g/mol and do
Recipe 2.
While other groups will start with the
weak base, HPO42-‐, in
the form of Na2HPO4
.7H2O(s), MM = 268.07 g/mol and do Recipe 3. We can only make buffers with
pH values around 7 with this wa + wb combo.
3. The math for Recipe 2: Star>ng with weak acid and adding hydroxide to make the buffer
pH = pKa + log( ) 0 mol wb + nOH-‐
nwa -‐ nOH-‐
We can make the weak base from the weak acid by adding strong base.
Hydroxide will convert some moles of the weak acid into weak base as shown in the equa;on in the white box.
Recipe 2 is used if you have the weak acid available, but not its conjugate weak base.
7 Info for
IntroducFon
So why is it that we have nOH-‐ in the top and
boTom?
This is the reac;on that takes place in Recipe 2:
HA + OH-‐ à H2O + A-‐
Henderson for Recipe 2:
3. The math for Recipe 2: Star>ng with weak acid and adding hydroxide to make the buffer
pH = pKa + log( ) 0 mol wb + nOH-‐
nwa -‐ nOH-‐
n = MV n = 0.0050 mol wa + wb
The Henderson for Recipe 2 can be simplified to this.
8 Info for
IntroducFon
pH = pKa + log( ) nOH-‐
nwa -‐ nOH-‐ Just like last week, we are supposed to make 50 mL of a 0.100 M buffer. This is taken to mean that we want the molarity
of wa plus wb to add up to 0.100 M.
3. The math for Recipe 2: Star>ng with weak acid and adding hydroxide to make the buffer
We subs;tute in nwa = 0.0050 mol because we are making our buffer
from weak acid. Suppose the assigned buffer was to have a pH = 7.00. The
rest is math.
Info for IntroducFon
pH = pKa + log( ) nOH-‐
nwa -‐ nOH-‐
pH = pKa + log( ) nOH-‐
0.0050 mol -‐ nOH-‐
7.00 = 7.198 + log( ) nOH-‐ 0.0050 mol -‐ nOH-‐
-‐0.198 = log( ) nOH-‐ 0.0050 mol -‐ nOH-‐ 9
3. The math for Recipe 2: Star>ng with weak acid and adding hydroxide to make the buffer
Remember with log numbers, all places aler the decimal are significant figures, so -‐0.198 has three sig
figs.
-‐0.198 = log( ) nOH-‐ 0.0050 mol -‐ nOH-‐
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0.634 = nOH-‐ 0.0050 mol -‐ nOH-‐
nOH-‐ = 0.00194 mol OH-‐ Make sure you can get this number for moles of hydroxide x
x
Math headache!!!
3. The math for Recipe 2: Star>ng with weak acid and adding hydroxide to make the buffer
Aler we’ve added 0.00194 moles hydroxide, we will have that many moles of weak base and the rest is unreacted weak acid. They add up to 0.0050 mol total. There will be no NaOH lel
– it’s the LR!
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The NaOH is 1.0 M. Using n = MV, we calculate that we
need to add 1.94 mL of 1.0 M NaOH.
nA-‐ = 0.00194 mol A-‐ nHA = 0.00306 mol HA ntotal = 0.00500 mol
nOH-‐ = MV
0.00194 mol = 1.0 mol L-‐1 x V
V = 0.00194 L = 1.94 mL
Woo-‐hoo! We get to use the Mohr pipet!
4. The math for Recipe 3: Star>ng with weak base and adding strong acid to make the buffer
Analogous to Recipe 2, we will start out with 0.0050 mol of weak base. (In Recipe 2 we started out with 0.0050 mol of wa.)
With Recipe 3 we start out with the weak base
and make the conjugate weak acid.
Info for IntroducFon
pH = pKa + log( ) nwb -‐
Henderson for Recipe 3 nH O+
3 0 mol wa + nH O+
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This is the reac;on that takes place in Recipe 3:
A-‐ + H3O+ à H2O + HA
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4. The math for Recipe 3: Star>ng with weak base and adding strong acid to make the buffer
And here is Henderson for Recipe 3.
13 Info for
IntroducFon
pH = pKa + log( ) nwb -‐ nH O+
3
nH O+
3
0.0050 mol wb -‐ nH O+
3 nH O+
3 pH = pKa + log( )
0.0050 mol wb -‐ nH O+
3 nH O+
3 7.00 = 7.198 + log( )
See if you can solve for moles of strong acid. I got 0.00306 mol. And I’m just a bird.
Aler we’ve added 0.00306 moles HCl, we will have that many moles of weak
acid and the rest is unreacted weak base. They add up to 0.0050 mol total. There will be no HCl lel –
it’s the LR!
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The HCl(aq) is 1.0 M. Using n = MV, we calculate that we
need to add 3.06 mL of 1.0 M HCl.
nA-‐ = 0.00194 mol
ntotal = 0.00500 mol
0.00306 mol HCl = 1.0 mol L-‐1 x V
V = 0.00306 L = 3.06 mL HCl
This is where I say “Woo-‐hoo! We get to use the Mohr pipet!”
4. The math for Recipe 3: Star>ng with weak base and adding strong acid to make the buffer
nHA = 0.00306 mol
nH O+ = 0.00306 mol HCl 3
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5. What we are doing in lab today.
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At your sta;on, you will have either NaH2PO4(s), MM =
119.98 g/mol and do Recipe 2, or you will have
Na2HPO4.7H2O(s), MM =
268.07 g/mol and do Recipe 3. With Recipe 2, we
start out with 0.00500 mol of the
weak acid, NaH2PO4(s). From the MM = 119.98 g/mol we can calculate
what mass we need. You should get 0.599 g
Then just follow the calcula;ons provided here for your recipe.
Make sure you understand everything.
With Recipe 3, we start out with 0.00500 mol of
the weak base, Na2HPO4
.7H2O(s), From the MM = 268.07 g/mol we can calculate what
mass we need.
You should get 1.340 g
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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) un;l it’s perfect.
Hmmm… What should I do if the pH is too low?
Remember this slide from last
week?
Info for IntroducFon
5. What we are doing in lab today.
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5. What we are doing in lab today.
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Stuff to do today….
① Wear your safety glasses. Dress for a mess.
② Take ;me wri;ng your introduc;on in your own words.
③ Carefully record observa;ons and details. ④ Use the pocket balances at your sta;on. They are good
to the hundredths place.
⑤ Make sure you are using the Mohr pipet correctly.
⑥ When your buffer is ready, ask your TA to check it for accuracy. She/he will ini;al the cover sheet if it’s ok. Then clean up and turn in your lab report!
⑦ The cover sheet summarizes everything that you need to include with your report.
⑧ Be nice to your pH probe.
6. Your Lab Report
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① First, the cover page with TA ini>als. ② Next, the trimmed copy pages from your lab
notebook stapled together. ③ Staple together. There are no printed pages. 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.
S>ck people inspired by xkcd cartoons by Randall Munroe
(www.xkcd.com)
Oh, one last thing before we turn out the lights… Did you no;ce that both Recipes 2 and 3 result in
making a buffer with 0.00306 mol HA and 0.00194 mol A-‐? (Check Slides 11 and 14.) PreTy
cool, huh?
Look Mom! I laid some eggs!
Happy Easter! See you in two weeks!