P3 - pKa of p-Nitrophenol Flashcards
what colour is p-Nitrophenol at acidic pH?
colourless, no absorbance
what colour is p-Nitrophenol at basic pH?
yellow
how do you prepare a buffer?
mixing a weak acid with its conjugate base
what is the pH of buffer NaH2PO4?
4.45
when 2 is not near A it is acidic
what is the pH of buffer Na2HPO4?
9.2
how do you determine p-Nitrophenol ion concentration at each pH?
Beer Lambert Law (A=ecl)
what is the molar absorption coefficient for p-Nitrophenol anion?
17600mol/cm
what is the pH when absorbance=0?
4.45
what is the pH when absorbance =0.9?
8.5
what does [A-] represent?
p-Nitrophenol anion
what does [HA] represent?
undissociated p-Nitrophenol
at acidic pH what does the solution consist mostly of?
[HA] undissociated p-Nitrophenol
what happens the solution when pH increases?
[HA] dissociates to [A-], p-Nitrophenol ion concentration increases
what happens when the ratio of [A-]/[HA] is >1?
the solution is turning yellow, pH is increasing
how is pKa calculated?
Henderson Hasselbach
pH = pKa + log([A-]/[HA])
what is pKa?
a number that shows how weak or strong an acid is
what does a low pKa indicate?
a strong acid with great ability to donate protons
high pka indicates…
a weak acid with a low ability to donate protons
what happens pH during reactions?
it is altered
how can you regulate pH during reactions?
buffers
how do buffers work?
they contain molecules that absorb/ release protons and electrons in response to pH change
what is a universal buffer?
phosphate
what is pKa?
a buffer solution of weak acid and conjugate base/ weak base and conjugate acid
what is the pH of sodium acetate and water? why?
8/9
protonation occurs due to acetate negative charge, strong base pulls protons from water, protons in solution decrease and pH increases
what is the acid dissociation constant?
The ratio of concentration of dissociated ions to the concentration of original acid (Ka)
What does it mean when Ph = pKa?
it tells us what pH range the solution acts as a H+ buffer
why is acetic acid a good buffer at low pH? (4.7)
It holds protons tightly - weak acid
low dissociation constant (more acetic acid than acetate in solution)
what is the relationship between pH and pKa?
pH = pKa + log[A-]/[HA]
what happens when a base is added to an acid buffer? example acetic acid and sodium hydroxide is added
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)
in what range does buffering occur?
roughly one unit above and below pKa value
does the concentration of buffer affect the buffering range?
no, it is independent of concentration
it has a greater buffering capacity
what happens acetic acid at high pH?
It loses a proton and has a negative charge - becoming acetate
what happens acetic acid at low pH?
it holds proton tightly (CH3COOH)
how many buffering regions does phosphate have?
3
why does phosphate have 3 different buffering regions?
it has 3 different pKas
what is the pKa of carboxylic acids?
5
what is pKa definition?
a value that describes the acidity of a molecule
what amino acids have 3 pKas?
aspartic acid, glutamic acid, lysine, arginine and histidine
why do amino acids have 2/3 pkas?
carboxyl group, ammonium group and side chain have different pKas
what is isoelectric point? pI?
the pH at which a particular molecule carries no net electrical charge
how is pI calculated?
the mean pKa values of molecule
how do you determine pI of a protein?
add all pKa values of each amino acid and calculate mean value
which AA has the highest pI?
Arginine(10.7), Lysine(9.7), Histidine (7.6)
what happens when pH is less than pI?
Excess of protons in solution - acidic
What happens when pH is greater than pI?
excess amount of OH- in solution, basic
high dissocation constant?
weak acid, dissociates into its original and anionic form
Describe a strong acid
low pKa
protons held loosley
high Ka
what buffer has greatest buffering capacity?
when pKa of acid = pH of solution
