P3 - pKa of p-Nitrophenol

Question 1

what colour is p-Nitrophenol at acidic pH?

Answer 1

colourless, no absorbance

Question 2

what colour is p-Nitrophenol at basic pH?

Answer 2

yellow

Question 3

how do you prepare a buffer?

Answer 3

mixing a weak acid with its conjugate base

Question 4

what is the pH of buffer NaH2PO4?

Answer 4

4.45
when 2 is not near A it is acidic

Question 5

what is the pH of buffer Na2HPO4?

Answer 5

9.2

Question 6

how do you determine p-Nitrophenol ion concentration at each pH?

Answer 6

Beer Lambert Law (A=ecl)

Question 7

what is the molar absorption coefficient for p-Nitrophenol anion?

Answer 7

17600mol/cm

Question 8

what is the pH when absorbance=0?

Answer 8

4.45

Question 9

what is the pH when absorbance =0.9?

Answer 9

8.5

Question 10

what does [A-] represent?

Answer 10

p-Nitrophenol anion

Question 11

what does [HA] represent?

Answer 11

undissociated p-Nitrophenol

Question 12

at acidic pH what does the solution consist mostly of?

Answer 12

[HA] undissociated p-Nitrophenol

Question 13

what happens the solution when pH increases?

Answer 13

[HA] dissociates to [A-], p-Nitrophenol ion concentration increases

Question 14

what happens when the ratio of [A-]/[HA] is >1?

Answer 14

the solution is turning yellow, pH is increasing

Question 15

how is pKa calculated?

Answer 15

Henderson Hasselbach
pH = pKa + log([A-]/[HA])

Question 16

what is pKa?

Answer 16

a number that shows how weak or strong an acid is

Question 17

what does a low pKa indicate?

Answer 17

a strong acid with great ability to donate protons

Question 18

high pka indicates…

Answer 18

a weak acid with a low ability to donate protons

Question 19

what happens pH during reactions?

Answer 19

it is altered

Question 20

how can you regulate pH during reactions?

Answer 20

buffers

Question 21

how do buffers work?

Answer 21

they contain molecules that absorb/ release protons and electrons in response to pH change

Question 22

what is a universal buffer?

Answer 22

phosphate

Question 23

what is pKa?

Answer 23

a buffer solution of weak acid and conjugate base/ weak base and conjugate acid

Question 24

what is the pH of sodium acetate and water? why?

Answer 24

8/9
protonation occurs due to acetate negative charge, strong base pulls protons from water, protons in solution decrease and pH increases

Question 25

what is the acid dissociation constant?

Answer 25

The ratio of concentration of dissociated ions to the concentration of original acid (Ka)

Question 26

What does it mean when Ph = pKa?

Answer 26

it tells us what pH range the solution acts as a H+ buffer

Question 27

why is acetic acid a good buffer at low pH? (4.7)

Answer 27

It holds protons tightly - weak acid low dissociation constant (more acetic acid than acetate in solution)

Question 28

what is the relationship between pH and pKa?

Answer 28

pH = pKa + log[A-]/[HA]

Question 29

what happens when a base is added to an acid buffer? example acetic acid and sodium hydroxide is added

Answer 29

the pH of the solution will stay in a range (3.76-5.76) until too much sodium hydroxide is added and pH changes dramatically, solution becomes basic (OH- takes protons)

Question 30

in what range does buffering occur?

Answer 30

roughly one unit above and below pKa value

Question 31

does the concentration of buffer affect the buffering range?

Answer 31

no, it is independent of concentration it has a greater buffering capacity

Question 32

what happens acetic acid at high pH?

Answer 32

It loses a proton and has a negative charge - becoming acetate

Question 33

what happens acetic acid at low pH?

Answer 33

it holds proton tightly (CH3COOH)

Question 34

how many buffering regions does phosphate have?

Answer 34

3

Question 35

why does phosphate have 3 different buffering regions?

Answer 35

it has 3 different pKas

Question 36

what is the pKa of carboxylic acids?

Answer 36

5

Question 37

what is pKa definition?

Answer 37

a value that describes the acidity of a molecule

Question 38

what amino acids have 3 pKas?

Answer 38

aspartic acid, glutamic acid, lysine, arginine and histidine

Question 39

why do amino acids have 2/3 pkas?

Answer 39

carboxyl group, ammonium group and side chain have different pKas

Question 40

what is isoelectric point? pI?

Answer 40

the pH at which a particular molecule carries no net electrical charge

Question 41

how is pI calculated?

Answer 41

the mean pKa values of molecule

Question 42

how do you determine pI of a protein?

Answer 42

add all pKa values of each amino acid and calculate mean value

Question 43

which AA has the highest pI?

Answer 43

Arginine(10.7), Lysine(9.7), Histidine (7.6)

Question 44

what happens when pH is less than pI?

Answer 44

Excess of protons in solution - acidic

Question 45

What happens when pH is greater than pI?

Answer 45

excess amount of OH- in solution, basic

Question 46

high dissocation constant?

Answer 46

weak acid, dissociates into its original and anionic form

Question 47

Describe a strong acid

Answer 47

low pKa protons held loosley high Ka

Question 48

what buffer has greatest buffering capacity?

Answer 48

when pKa of acid = pH of solution