Experiment Questions: Titration

Question 1

To prepare a standard solution of sodium carbonate.Why is sodium carbonate suitable as a primary standard? Distinguish between a primary standard and a standard solution.

Answer 1

Sodium carbonate is suitable as a primary standard because it can be obtained in a pure and stable
state and it dissolves easily in water.
A primary standard is a substance which can be obtained in a stable, pure and soluble solid form so
that it can be weighted out and dissolved in water to give a solution of accurately know
concentration.
A standard solution is a solution of precisely known concentration.

Question 2

To prepare a standard solution of sodium carbonate. Why is it necessary to slowly add the solid sodium carbonate to the water in the beaker?

Answer 2

Adding the sodium carbonate slowly to the water helps prevent lumps being formed in the solution.

Question 3

To prepare a standard solution of sodium carbonate. State two benefits of a volumetric flask for making up the solution.

Answer 3

(1) The volume of the solution in the volumetric flask is accurately known.
(2) The design of the volumetric flask allows it to be shaken easily to ensure a homogenous mixture,
i. e. to ensure that the solution is uniform throughout.

Question 4

To use the standard solution of sodium carbonate solution prepared in experiment 13.1 to standardise a given hydrochloric acid solution. Why is it not recommended procedure to pipette directly from the volumetric flask?

Answer 4

This is not recommended because any impurities on the pipette would contaminate the entire solution in the volumetric flask.

Question 5

To use the standard solution of sodium carbonate solution prepared in experiment 13.1 to standardise a given hydrochloric acid solution. Why is it important not to add too much indicator?

Answer 5

Indicators are either weak acids or weak bases and adding too much indicator could affect the
accuracy of the result.

Question 6

To use the standard solution of sodium carbonate solution prepared in experiment 13.1 to standardise a given hydrochloric acid solution. Why do we have to standardise HCl solution?

Answer 6

The HCl solution must be standardised because HCl itself is not a primary standard, i.e. it is not possible to take an accurately known amount of HCl and dissolve it in a fixed volume of water to give a solution of known concentration.

Question 7

To use the standard solution of sodium carbonate solution prepared in experiment 13.1 to standardise a given hydrochloric acid solution. What is the purpose of carrying out the titration more than once?

Answer 7

Carrying out the titration more than once gives a more accurate titration figure since the average of two readings agreeing to within 0.1 cm3 is used in the calculations.

Question 8

To use the standard solution of sodium carbonate solution prepared in experiment 13.1 to standardise a given hydrochloric acid solution. Why is the result of the first titration usually ignored?

Answer 8

It is ignored because it is likely to be inaccurate, since the purpose of the first titration is to tell you
approximately where the end point lies.

Question 9

To use the standard solution of sodium carbonate solution prepared in experiment 13.1 to standardise a given hydrochloric acid solution. Name the gas given off during the titration.

Answer 9

Carbon dioxide. (Na2CO3 + 2HCl Æ 2NaCl + H2O + CO2)

Question 10

To make up an approximate 0.1 M solution of sodium hydroxide, to standardise it (i.e. obtain the exact concentration) with a standard hydrochloric acid solution and hence to prepare a sample of sodium chloride. Why is it important not to allow solid sodium hydroxide to come in contact with your skin?

Answer 10

Solid sodium hydroxide is corrosive and causes severe burns.

Question 11

To make up an approximate 0.1 M solution of sodium hydroxide, to standardise it (i.e. obtain the exact concentration) with a standard hydrochloric acid solution and hence to prepare a sample of sodium chloride. How is it possible to standardise the NaOH in this experiment when the HCl is not a primary standard?

Answer 11

Even though the HCl is not a primary standard, the concentration of the solution is known from the
previous experiment, i.e. the HCl solution is a standard solution.

Question 12

To make up an approximate 0.1 M solution of sodium hydroxide, to standardise it (i.e. obtain the exact concentration) with a standard hydrochloric acid solution and hence to prepare a sample of sodium chloride. Why is the indicator left out when performing a titration to isolate a sample of the sodium chloride?

Answer 12

The indicator would contaminate the sample

Question 13

To determine the concentration of ethanoic acid (acetic acid) in vinegar. Why is it important to use colourless vinegar rather than brown vinegar in this experiment?

Answer 13

Because the brown colour of the vinegar would mask the colour change at the end point.

Question 14

To determine the concentration of ethanoic acid (acetic acid) in vinegar. Give the name and formula of the acid commonly found in vinegar.

Answer 14

Ethanoic acid (acetic acid), CH3COOH.

Question 15

To determine the concentration of ethanoic acid (acetic acid) in vinegar. Why is the vinegar diluted?

Answer 15

If undiluted vinegar were used a very large quantity of sodium hydroxide solution would be required
to neutralise it,

Question 16

To determine the concentration of ethanoic acid (acetic acid) in vinegar. What colour change is observed in this experiment?

Answer 16

Using phenolphthalein indicator, the colour change is from pink to colourless.

Question 17

To determine the concentration of ethanoic acid (acetic acid) in vinegar. Name the salt formed in this titration.

Answer 17

The salt formed is called sodium ethanoate

Question 18

To determine the concentration of ethanoic acid (acetic acid) in vinegar. Give two safety precautions you would take when carrying out the experiment.

Answer 18

Wear safety glasses. Use a pipette filler. Wear a laboratory coat.

Question 19

To determine the percentage of water of crystallisation in hydrated sodium carbonate (washing soda). What is water of crystallisation?

Answer 19

Water of crystallisation is water that makes up part of the crystal structure of a compound.

Question 20

To determine the percentage of water of crystallisation in hydrated sodium carbonate (washing soda). What colour change is observed during this titration?

Answer 20

The colour changes from yellow (orange) to pink (for methyl orange indicator).

Question 21

To use the standard solution of sodium carbonate solution prepared in experiment 13.1 to standardise a given hydrochloric acid solution. Why are the sides of the conical flask washed down with deionised water

Answer 21

To make sure all of the acid in the burette reacts to the base.

Question 22

To use the standard solution of sodium carbonate solution prepared in experiment 13.1 to standardise a given hydrochloric acid solution. Why can any amount of desionsied water be added to the conical flask during any stage of the titration.

Answer 22

It will not affect the number of moles in the sodium carbonate solution

Question 23

Why is phenolphthalein used in the experiment?

Answer 23

Because the titration has a weak acid and a strong base

Question 24

Why is methyl orange indicator used?

Answer 24

Because the titration has a strong acid but a weak base