Core Practical 9

Question 1

What is Ka?

Answer 1

acid dissociation constant which measures quantitatively the strength of acid in solution.

Question 2

Why are acid-base indicators used?

Answer 2

To detect when a reaction reaches it’s equivalence point. indicator chosen should have end point = equivalence point

Question 3

Why does a pH probe need to be calibrated?

Answer 3

so that for each pH reaction each value is accurate

Question 4

How do you calibrate a pH probe?

Answer 4

Submerge probe in buffer solutions of 3 different pH’s around 4, 7, 10 each time pressing the calibrate button

Question 5

What is accuracy?

Answer 5

The more accurate the data the closer it is to the actual value

Question 6

how do you carry out a titration to calculate pH

Answer 6

Titrate 25 cm​3​ of the acid being tested against NaOH, use phenolphthalein indicator. After titration, add another 25 cm​3​ of ethanoic acid to the same flask.
3. Use pH meter to find pH. pH will equal pKa because exactly half of the acid has been neutralised so this is the half-equivalence point.
[i.e. [A​−​] = [HA], therefore Ka = [H​+​], and so pKa = pH.]

Question 7

How can you ensure greater accuracy when using the pH probe?

Answer 7

Wash with deionised water between readings to remove ions attached to the bulb.