(blue) buffers indicators pH titation curves

Question 1

what is a buffer

Answer 1

can resist changes in pH despite the addition of small amounts of acid or alkali.

Question 2

a buffer is a solution containing

Answer 2

A weak acid and the salt of that weak acid eg CH3COOH(aq) + CH3COONa(aq)

OR: A weak alkali and the salt of that weak alkali. eg NH4OH(aq) + NH4Cl(aq)

Question 3

how does a buffer work

Answer 3

Buffers are examples of equilibrium systems. A typical buffer consists of a weak acid AND its salt, for buffers below 7 or a weak base AND its salt for buffers above 7

Question 4

in a Ethanoic acid (acid) and Sodium ethanoate (salt)

If you add Acid to the system, What happen?

If you add alkali to the system what happens?

Answer 4

acid :
A buffer contains a large reservoir of CH3COOH and CH3COO- . On addition of a small amount of acid (H+) the equilibria position shifts to the LHS the H+ reacting with the CH3COO- therefore the ration of [H+] / [CH3COO-] hardly changes and pH stays roughly constant.

base

A buffer contains a large reservoir of CH3COOH and CH3COO- . On addition of a small amount of alkai (OH-) the H+ ions react with the OH- ions. the equilibria position shifts to the RHS, more acid then dissociates to maintain the concentration of H+ , therefore the ration of [H+] / [CH3COO-] hardly changes and pH stays roughly constant.

Question 5

expalin how this :
CH3COOH CH3COO- + H+

acts as a buffer solution

Show that the H+ concentration does not vary.

Answer 5

On addition of H+ ions the above equilibrium shifts to the LHS, H+ combine with ethanoate ions to form ethanoic acid so removing the added H+ from solution and keeping [H+] relatively constant.

On addition of OH- they combine with the H+ in solution. The [H+] is kept relatively constant by dissociation of more ethanoic acid (equilibrium shifts to RHS).

Question 6

[CH3COOH(aq)]original = [CH3COOH(aq)]equilibrium

why can we make this assumption

Answer 6

Ethanoic acid is a weak acid with very little/ no dissociation, dissociation is further suppressed by the high concentration of CH3COO- from the salt.

because its also soluable

Question 7

what is the Ka expression

Answer 7

Ka = [salt] x [H+] / [acid]

Question 8

how do you find pH from Ka

Answer 8

when salt = acid , which is at half equivulance point then just minus log it and then you get pH

Question 9

what is the equivulance point

Answer 9

The equivalence point is the point at which: no. moles H+(aq) = no. moles OH-(aq)

Question 10

what is an indicator

Answer 10

An indicator is a weak acid or a weak base, which has one colour in the undissociated form and a different colour as a dissociated ion.

Question 11

what is the ideal indicator

Answer 11

The ideal indicator for a given acid - base titration is one that is in the middle of its colour change at the pH of the equivalence point of the titration. i.e. end point = equivalence point