pH + buffers

Question 1

what is pH?

Answer 1

A measure of H+ concentration

Question 2

What is the equation for pH?

How can this be rearranged to find the H+ concentration?

Answer 2

pH = -log10 [H+]
[H+] = 10^(-pH)

Question 3

Whats the difference between strong and weak acids?

Answer 3

Strong acids almost fully dissociate and have an equilibrium at the far right.
Weak acids partially dissociate and has much of the non-ionised acid remaining at equilibrium

Question 4

What are the basic steps to calibrate a pH meter?

Answer 4

• rinse and dry electrode between each step
• place in pH 7.0 buffer
• place in acidic or alkaline buffer
• place back into pH 7.0 buffer
• should read within 0.05 units of 7.0

Question 5

What is the equilibrium constant (Ka)?

Answer 5

Ka = [A-][H+] / [HA]

Question 6

What is the pKa?

Answer 6

The log10 of the Ka.

pKa = -logKa

Question 7

What is the Henderson-Hasselbalch equation?

Answer 7

pH = pKa + log10[A-]/[HA]

Question 8

What is the main buffer in the blood and interstitial fluid?

Answer 8

The bicarbonate buffer system.

Question 9

What is the overall equation of the bicarbonate buffer system?

Answer 9

CO2 + H20 -> HCO3- + H+

Question 10

Why is phosphate a good buffer in biochemistry?

Answer 10

It has three pKa values due to it’s three ionisable groups

Question 11

What is the significance of a buffer’s pKa?

Answer 11

The buffer is most resistant to changes in the pH at the pKa where the acid and base concentrations are equal.

Question 12

What does it mean when a molecule is polar?

Answer 12

It has an uneven distribution of charge

Question 13

What is a hydrogen bond?

Answer 13

When a hydrogen atom is shared between two electronegative atoms like O + N.

Question 14

What does the hydrogen bonding in water allow it to do?

Answer 14

Allows it to act as a much larger molecule as they are constantly making and breaking bonds with other water molecules

Question 15

What is Gibbs free energy? What does it tell us?

Answer 15

Tells us if a reaction will occur spontaneously.
If G > 0 non spontaneous
If G = 0 reaction at equilibrium
If G < 0 reaction is spontaneous

Question 16

What is the equation for Gibbs free energy?

Answer 16

△G = △H - T x △S

H = enthalpy
T = temperature
S = entropy

Question 17

What happens to water at the surface of a hydrophobic protein?

Answer 17

The water molecules exchange fewer hydrogen bonds and form ordered shells with a low entropy around the protein

Question 18

Why do hydrophobic proteins fold?

Answer 18

They fold to reduce the surface area of hydrophobic protein. This allows water to form more hydrogen bonds and increases entropy therefore decreasing Gibbs free energy which is favourable

Question 19

Show the dissociation of pure water

Answer 19

H20 <> H+ + OH-

[H+] = [OH-]

At 25C [H+] = 10^7 mol.L^-1

Question 20

What is Le Chatelier’s Principle?

Answer 20

If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change

Question 21

What substances can act as buffers?

Answer 21

Any weak acid (or base) around its pKa

Amino acids by accepting or donating H+

Question 22

A proton exists in a solution as?

Answer 22

A hydronium ion