Chapter 21 Buffers

Question 1

What is a buffer?

Answer 1

A buffer is a solution that minimises changes in pH when small amounts of acid or base are added

Question 2

What are the two ways of making a acidic buffer?

Answer 2

• Mix a weak acid with the salt of its conjugate base

- Mix an excess of weak acid with a strong alkali

Question 3

Describe how conjugate pairs resist changes in pH if you add a small amount of acid

Answer 3

If you add a small amount of acid, the H+ concentration increases

Most of the extra H+ ions will combine with the base-pair in solution that accepts protons

This shifts the equilibrium to the left, reducing the H+ concentration to close to its original value, so the pH doesn’t change much

Question 4

Describe how conjugate pairs resist changes in pH if you add a small amount of alkali

Answer 4

If a small amount of alkali is added, the OH- concentration increases

Most of the extra OH- ions react with H+ ions to form water – Removing the H+ ions from the solution

This causes the acid pair to dissociate to form H+ ions – shifting the equilibrium to the right

The concentration of H+ increases until its close to its original value, so the pH doesn’t change much

Question 5

What two assumptions are made when calcualting the pH of a Buffer solution?

Answer 5

• The salt of the conjugate base is fully dissociated, so assume that the equilibrium concentration of A- is the same as the inital concentration of the salt
• HA is only slightly dissociated, so assume that its equilibrium concentration is the same as its inital concentration

Question 6

How can you measure pH?

Answer 6

Using a pH Meter

Question 7

Why are pH curves useful?

Answer 7

pH curves help decide which indicator to use