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PSCM PRACTICAL MCQ > P3 - pKa of p-Nitrophenol > Flashcards

P3 - pKa of p-Nitrophenol Flashcards

1
Q

what colour is p-Nitrophenol at acidic pH?

A

colourless, no absorbance

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2
Q

what colour is p-Nitrophenol at basic pH?

A

yellow

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3
Q

how do you prepare a buffer?

A

mixing a weak acid with its conjugate base

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4
Q

what is the pH of buffer NaH2PO4?

A

4.45
when 2 is not near A it is acidic

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5
Q

what is the pH of buffer Na2HPO4?

A

9.2

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6
Q

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

A

Beer Lambert Law (A=ecl)

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7
Q

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

A

17600mol/cm

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8
Q

what is the pH when absorbance=0?

A

4.45

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9
Q

what is the pH when absorbance =0.9?

A

8.5

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10
Q

what does [A-] represent?

A

p-Nitrophenol anion

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11
Q

what does [HA] represent?

A

undissociated p-Nitrophenol

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12
Q

at acidic pH what does the solution consist mostly of?

A

[HA] undissociated p-Nitrophenol

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13
Q

what happens the solution when pH increases?

A

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

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14
Q

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

A

the solution is turning yellow, pH is increasing

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15
Q

how is pKa calculated?

A

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

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16
Q

what is pKa?

A

a number that shows how weak or strong an acid is

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17
Q

what does a low pKa indicate?

A

a strong acid with great ability to donate protons

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18
Q

high pka indicates…

A

a weak acid with a low ability to donate protons

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19
Q

what happens pH during reactions?

A

it is altered

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20
Q

how can you regulate pH during reactions?

A

buffers

21
Q

how do buffers work?

A

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

22
Q

what is a universal buffer?

A

phosphate

23
Q

what is pKa?

A

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

24
Q

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

A

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

25
Q

what is the acid dissociation constant?

A

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

26
Q

What does it mean when Ph = pKa?

A

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

27
Q

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

A

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

28
Q

what is the relationship between pH and pKa?

A

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

29
Q

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

A

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)

30
Q

in what range does buffering occur?

A

roughly one unit above and below pKa value

31
Q

does the concentration of buffer affect the buffering range?

A

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

32
Q

what happens acetic acid at high pH?

A

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

33
Q

what happens acetic acid at low pH?

A

it holds proton tightly (CH3COOH)

34
Q

how many buffering regions does phosphate have?

A

3

35
Q

why does phosphate have 3 different buffering regions?

A

it has 3 different pKas

36
Q

what is the pKa of carboxylic acids?

A

5

37
Q

what is pKa definition?

A

a value that describes the acidity of a molecule

38
Q

what amino acids have 3 pKas?

A

aspartic acid, glutamic acid, lysine, arginine and histidine

39
Q

why do amino acids have 2/3 pkas?

A

carboxyl group, ammonium group and side chain have different pKas

40
Q

what is isoelectric point? pI?

A

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

41
Q

how is pI calculated?

A

the mean pKa values of molecule

42
Q

how do you determine pI of a protein?

A

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

43
Q

which AA has the highest pI?

A

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

44
Q

what happens when pH is less than pI?

A

Excess of protons in solution - acidic

45
Q

What happens when pH is greater than pI?

A

excess amount of OH- in solution, basic

46
Q

high dissocation constant?

A

weak acid, dissociates into its original and anionic form

47
Q

Describe a strong acid

A

low pKa
protons held loosley
high Ka

48
Q

what buffer has greatest buffering capacity?

A

when pKa of acid = pH of solution

PSCM PRACTICAL MCQ (7 decks)

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