Titrations

Question 1

Method for a titration 6-9

Answer 1

1. Use the pipette to measure 25cm3
   of sodium hydroxide into the conical flask.
2. Place the conical flask on a white tile.
3. Fill the burette with sulphuric acid using a funnel.
4. Record the initial reading of acid in the burette.
   - take readings from the bottom of the meniscus.
5. Add a 5 drops of indicator in this case phenolphthalein to the conical flask.
6. Slowly open the burette tap while swirling the conical flask.
7. Add acid drop-by-drop near the endpoint.
   - At this point the colour will start to change slightly.
8. Close the burette when a colour change occurs in phenolphthalein.
   - The solution turns from pink to colourless.
9. Record the final reading of acid in the burette and calculate the titre.

Question 2

How to record results for a titration 3-4

Answer 2

10. Repeat until you have concordant results.
    - These are within 0.1cm3
    of each other.
11. Present results in a table and calculate the mean titre discarding any anomalies when calculating the mean.
12. Calculate the number of moles of sodium hydroxide used in the titration.
13. In the balanced equation the ratio between sodium hydroxide and sulphuric acid is 2:1.
    Therefore to find out the moles of sulphuric acid divide the moles of sodium hydroxide
    by 2.
14. Use the formula [concentration= moles/volume (mean titre volume)] to work out the
    concentration of sulphuric acid.

Question 3

List the apparatus required to carry out a

titration6

Answer 3

• Burette
• Conical flask
• Pipette and pipette filler
• Funnel
• Clamp and stand
• White tile

Question 4

Why should a white tile be used in a titration experiment?

Answer 4

The white tile is placed under the
reacting mixture, making the colour
change easier to see

Question 5

Why might a volumetric pipette and pipette filler be used to measure 25cm3 rather than a measuring cylinder

Answer 5

A volumetric pipette will measure 25cm3
more accurately so it is usually used for
titrations

Question 6

What is the purpose of repeating a titration?

Answer 6

To allow a mean titre to be calculated,

reducing the effect of random error

Question 7

What colour is phenolphthalein in acid and alkali?2

Answer 7

acid- colourless

alkali-pink

Question 8

What safety precautions should be

undertaken when carrying out a titration?3

Answer 8

• Wear eyewear and be aware of the dangers of the chemicals used, e.g. H2SO4 is corrosive, NaOH is an
  irritant
• The burette should be filled below eye level so there’s no risk of it spilling and splashing your face
• Any broken glassware needs to be cleared immediately

Question 9

Why are burettes used for titrations?

Answer 9

Burettes allow the solution to be added

drop by drop in very small quantities

Question 10

Apart from phenolphthalein, what other indicator could be used in a titration?
What is the colour change?

Answer 10

Methyl Orange: Red (acid) to yellow (alkali)

Litmus Blue: Red (acid) to blue (alkali)

Question 11

If you know the volume of acid required to neutralise an
alkali, how could you calculate the concentration of the acid,
given the alkali concentration and volume?3

Answer 11

Calculate the number of moles of the alkali using the known volume
and concentration
- Use the chemical equation to work out the ratio of acid and alkali
that react and hence work out how many mole of acid have reacted
- Divide the moles of acid by the volume used in neutralisation

Question 12

If you know the concentration of an acid in mol/dm3 how could you convert this to /dm3?

Answer 12

Multiply the concentration in mol/dm3
by the molar mass (Mr) of the acid to give
the value in g/dm3