Experiment9 22March2018*...

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.

3

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.

4

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.

6

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:


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.


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.



nwa -‐ nOH-‐


0.0050 mol -‐ nOH-‐

7.00 = 7.198 + log( ) nOH-‐ 0.0050 mol -‐ nOH-‐

-‐0.198 = log( ) nOH-‐ 0.0050 mol -‐ nOH-‐ 9


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-‐

10

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!!!


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!

11

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.


pH = pKa + log( ) nwb -‐

Henderson for Recipe 3 nH O+

3 0 mol wa + nH O+

3

This is the reac;on that takes place in Recipe 3:

A-‐ + H3O+ à H2O + HA

12


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!

14

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!”


nHA = 0.00306 mol

nH O+ = 0.00306 mol HCl 3

15

5. What we are doing in lab today.

15

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

16

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?



17 17


17

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

18

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!

Experiment9 22March2018*...

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