Buffer Solutions

Question 1

What is a buffer solution

Answer 1

Solution which resists changes in pH when small amounts of acid/alkali are added, used in shampoo/water in swimming pools eg

Question 2

What does a typical buffer mixture consist of and how does this work

Answer 2

Solution of a weak acid and it’s salt/CB eg ethanoic acid and sodium ethanoate, must be plenty of both the acid and it’s salt so that any change does not affect pH as concen of HA and A- so high will remain fairly constant

Question 3

What happens when a little H+ added to buffer

Answer 3

Temporarily increases concentration of H+ so equilibrium shifts towards left hand side of equation to reduce H+ ion concen which counteracts the change as backward reaction favoured and plenty of base ions from the salt to react with H+ ions to produce acid

Question 4

What happens when a little OH- added

Answer 4

OH- concen increases, primarily reacts with acid to make water and salt, also reacts with H+ to make water- as hydrogen ion concen decreased equilibrium will move to right to restore the H+ lost, the acid required for this is readily available due to large excess as so little of the ethanoic acid dissociated in first place

Question 5

Ionic equations for buffer solution of ethanoic acid and sodium ethanoate

Answer 5

CH3COOH reversible CH3COO- + H+
CH3COONa reversible CH3COO- + Na+
Lots of CH3COOH which barely dissociates and CH3COO- from sodium ethanoate which fully ionises

Question 6

How to calculate pH of buffer

Answer 6

[H+]= Ka x HA/A-, uses equilibrium law as Ka = [H+][A-]/ [HA] but rearranged

Question 7

When do all the negative ions come from in buffer

Answer 7

From the added salt as it fully ionises, weak acid only slightly

Question 8

Does [A-]= [H+] for buffer solutions

Answer 8

No as [A-] coming from weak acid very small compared to that coming from its conjugate base

Question 9

In pH of buffer solutions where does concen of A- come from

Answer 9

Same as conjugate base concen as it completely ionises

Question 10

In pH of buffer solutions where does the concentration of HA come from

Answer 10

Same as original acid as so little dissociates

Question 11

Formula of potassium ethanoate

Answer 11

KCH3COO

Question 12

What happens to number of moles of HA and A- when hydroxide added

Answer 12

HA decreases by number of moles of sodium hydroxide as reacts to form salt and A- increases by number of moles of sodium hydroxide

Question 13

How to calculate pH of buffer solution with weak acid and strong base

Answer 13

[H+]= ka no moles of HA - OH- over total volume / [OH-] using total volume