9- Determing ka for Weak Acid

Question 1

Describe method used to calculate Ka of weak acid - CP9

Answer 1

1- Calibrate PH metre by submerging in Buffer solution
2- Pipette ethanoic acid into 250cm3 conical flask
3- Rinse + Fill buttered with NaOH
4- Add drops of Phenolphthalein indicator
5- titration ethanoic acid against NaOH = until solution turns Pale Pink
6- Add Small amount of Ethanoic acid into flask and measure PH

Question 2

What is Ka and what does it measure

Answer 2

Acid dissociation constant
= measures strength of acid in solution/How far it will ionise

Question 3

What do indicators detect for

Answer 3

When reaction reaches equivalence point = colour change (End point)

Question 4

At half the equivalence point what can we assume

Answer 4

HA = A-

Thefore Ka = H+
And PKa = PH

Question 5

Diagram of PH curve of weak acid + Strong base to determine Ka of weak acid

Answer 5

Question 6

Show the equation happening in CP9 where ethanoic acid is titrated against NaOH

Answer 6

CH3COOH + NaOH = CH3COO(-)Na(+) + H2O

HA =. HA + H+