Ionisation of Water, pH & Buffers

Question 1

Polar molecules in water

Answer 1

Have a high proportion of polar or ionic groups

This makes them hydrophilic as the polar groups increase hydrogen bonding and solubility

Question 2

Non-polar groups in water

Answer 2

Have few or no polar/ionic groups

They are hydrophobic as they minimise contact with water

Have an entropic effect = water around hydrophobic molecules have reduced mobility

Question 3

What are amphipathic molecules?

Answer 3

Have a polar and non-polar side

Aggregate into micelles, monolayers and vesicles

Question 4

Equilibrium constant of water

Answer 4

1.8 x 10-16

Kw = 10-14

Question 5

pH of aqueous solutions

Answer 5

Acidic = 3

Neutral = 7

Basic = 10

Question 6

The Henderson-Hasselbalch equation

Answer 6

pH = pKa + log10(Ac-/HAc)

If [Ac-] = [HAc] then pH is equal to pKa

Question 7

What happens if the solution is near the pKa value?

Answer 7

Adding small amounts of acid or base does not change the pH by much

Question 8

Function of buffers

Answer 8

Stabilises weak acids or bases

Question 9

When is the titration curve the flattest?

Answer 9

Near where pH = pKa

Question 10

What happens when the pH is equal to the pKa?

Answer 10

The acid has the strongest buffer capacity so has the ability to resist changes in pH when an acid or base is added

Question 11

Buffers in the lab

Answer 11

The pK is 7.5

Use to buffer between 6.5 and 8.5