Topic 12

Question 1

Acid base equation

Answer 1

HA+ H20—> H30+ + A-

Question 2

Acid base equation 2

Answer 2

B + H20 —> BH+ + OH-

Question 3

Examples of strong acids

Answer 3

HCl H2SO4 HNO3 H3PO4

Question 4

Water is amphoteric what does that mean and explain the circumstances

Answer 4

Water behaves as a base when acid is added to it

Question 5

Ammonia reacts with acids to make ammonium salts but no water

Answer 5

2NH3+H2SO4 —> (NH4)2SO4

Question 6

Calculate the pH of Diprotic and triprotic acids

Answer 6

H2SO4 is diprotic so pH=-log(2×H+)
H3PO4 is triprotic so pH=-log(3H+)

Question 7

How to find the pH of a base

Answer 7

Use Kw=[H+][OH-] where Kw is 1×10^(-14)

Question 8

What is the Ionic product of water(Kw)

Answer 8

The value of Kw is the same in a solution
Kw= 1×10^(-14)
Value changes if the temperature changes

Question 9

Explain how the carbonic acid-hydrogencarbonate(H2CO3 and HCO3-) controls the pH of blood

Answer 9

CO2 dissolves in blood forming carbonic acid(H2CO3)
Equilibrium shift to the right favouring H+ and HCO3- and produces more H+ ions
Large reservoir of HCO3- reacts with H+ ions

Question 10

How to make an Acidic buffer

Answer 10

1. React a weak acid with the salt of its conjugate base e.g ethanoic acid and sodium ethanoate
2. React excess weak acid and a strong base e.g ethanoic acid and NaOH

Question 11

How to make basic buffers(keep pH above 7)

Answer 11

Made from weak base and its salt e.g ammonia and ammonium chloride

Question 12

Calculate the pH of the strong base 0.1 mol dm-3 NaOH

Assume complete dissociation.

Answer 12

Kw = [H+ (aq)][OH- (aq)] = 1x10-14
H+] = Kw / [OH- (aq)] = 1x10-14 / 0.1 = 1x10-13 mol dm-3
pH = - log[1x10-13 ] =13.00

Question 13

What are the 2 assumptions when calculating pH of a weak acid

Answer 13

1) H+]eqm = A-] eqm because they have dissociated
according to a 1:1 ratio.
2) As the amount of dissociation is small we assume that the initial
concentration of the undissociated acid has remained constant.
So [HA (aq) ] eqm = [HA(aq) ]
initial

Question 14

Explain the effects of adding H+ on the carbonic acid hydrogencarbonate buffer

H2CO3(aq) ⇌ H+(aq) +HCO3-(aq)

Answer 14

Adding alkali reacts with H+ with the equation
H+ + OH- → H2O
so the above equilibrium would shift right forming new H+(as H+ has decreased) and more HCO3

Question 15

Explain the effects of adding acid on the carbonic acid hydrogencarbonate equilibrium

H2CO3(aq) ⇌ H+(aq) + HCO3–(aq)

Answer 15

Adding acid shifts the above equilibrium left.
The reaction is
H+ + HCO3– → H2CO3

Question 16

When should and shouldn’t phenolphthalein be used as an indicator in a titration

Answer 16

Only use when there is strong bases(not weak bases)

Colour change:
Colourless acid→ pink alkali

Question 17

Explain how the Carbonic acid hydrogencarbonate buffer system restored pH of the blood after exercise

H2CO3→ H+ + HCO3-

Answer 17

There is a large reservoir of HCO3- and H2CO3(HA and A-)

When H+ ion increases it reacts with HCO3- to form H2CO3 restoring pH

Question 18

How to measure Ka practical

Answer 18

Titrate ethanoic acid(WA) with sodium hydroxide(SB)
Measure pH at regular intervals
Plot pH against volume
Use graph to find pH at half equivalence point
Half equivalence neutralisation point is when pH=pKa
Measure pH of resultant mixture