CPAC 09

Question 1

What is the method for determining the Ka of a weak acid?

Answer 1

1) calibrate pH meter
2) pipette 25cm3 of 0.1moldm-3 ethanoic acid solution into 250cm3 conical flask
3) fill burette with sodium hydroxide solution
4) add 2 or 3 drops of phenolphthalein to conical flask
5) titrate ethanoic acid solution with NaOH solution until mixture just turns pink
6) pipette a further 25cm3 of 0.1moldm-3 ethanoic acid solution into 250cm3 conical flask
7) record pH of solution

Question 2

What are the sources of uncertainity in this experiment?

Answer 2

• end point difficult to identify exactly

- inaccuracy of burette readings

Question 3

How do you overcome sources of uncertainity?

Answer 3

• use white tile to clearly identify end point

- read burette readings from bottom of meniscus

Question 4

Why does the pH probe need to be calibrated?

Answer 4

so that the pH values are accurate for each reading

Question 5

How do you calibrate a pH probe?

Answer 5

submerge pH in different buffer solutions of 3 different pHs including pH 7 and usually 4 and 10 each time pressing the calibrate button