Titration - standardisation of HCl solution using a standard solution of anhydrous Na₂CO₃

Question 1

Apparatus needed

Answer 1

(B,C,F,P,P,R,S,W,W)

• Burette
• Conical flask
• Funnel
• Pipette
• Pipette filler
• Retort stand
• Safety glasses
• Wash bottle w deionised water
• White tile

Question 2

Materials needed

Answer 2

• .1 mol/l Na₂CO₃
• HCl solution
• Methyl orange indicator

Question 3

What is the first step?

Answer 3

Pipette, burette and conical flask were washed with deionised water

Question 4

What was the burette washed with?

Answer 4

Some of the HCl solution

Question 5

Why was the burette washed with some of the HCl solution?

Answer 5

To prevent dilution of the solution

Question 6

What happens to the burette after it has been washed with some of the HCl solution?

Answer 6

It was clamped vertically using the retort stand

Question 7

What happens after the burette gets clamped?

Answer 7

Using a funnel the burette was then filled with the HCl solution.

Question 8

Why was the tap opened briefly when the burette was filled with the HCl solution?

Answer 8

To allow the bit below the tap to fill.

Question 9

What happened after the funnel was used?

Answer 9

Funnel was removed and the solution was allowed to run off until the bottom of the meniscus was on the zero graduation mark.

Question 10

What happened after the funnel was removed?

Answer 10

Some of the sodium carbonate solution was poured into a clean dry beaker

Question 11

What happened after some of the sodium carbonate solution was poured into a clean dry beaker?

Answer 11

The pipette was rinsed out with some of this solution.

Question 12

Why was the pipette rinsed out with some of the solution already made?

Answer 12

To prevent a change in concentration.

Question 13

What happened after the pipette was rinsed out with some solution?

Answer 13

25cm³ of the sodium carbonate solution were pipetted into a clean conical flask

Question 14

What was added to the conical flask after the sodium carbonate solution?

Answer 14

3 drops of methyl orange indicator.

Question 15

What colour did the solution become after the methyl orange indicator was added?

Answer 15

Yellow (in alkaline solution)

Question 16

What happened after the indicator was added?

Answer 16

The conical flask was placed on a white tile

Question 17

Why was the conical flask placed on the white tile after the indicator was added?

Answer 17

So an accurate colour change would be seen to indicate the end-point.

Question 18

What happened after the conical flask was placed on the white tile?

Answer 18

The tap on the burette was opened and some of the HCl solution was added to the conical flask.

Question 19

What happened after some of the HCl solution was added to the conical flask?

Answer 19

The conical flask was swirled continuously

Question 20

What happened during the titration?

Answer 20

It was washed down with deionised water

Question 21

Why was the conical flask washed down with deionised water during the titration?

Answer 21

To ensure that all the HCl molecules react with the sodium carbonate in the conical flask.

Question 22

The titration was continued until the solution turned…..

Answer 22

Pink

Question 23

What was noted on the burette after the titration ended?

Answer 23

The titre was noted

Question 24

What was the first titration called?

Answer 24

The rough titration result

Question 25

Why is the rough titration important

Answer 25

So that the subsequent (next) titres can be more accurate.

Question 26

Which titration is ignored when calculating the titration?

Answer 26

The first one

Question 27

How many more times was the titration repeated?

Answer 27

2

Question 28

What happened in the next two titrations?

Answer 28

The HCl solution was added drop by drop as the end-point neared.

Question 29

What were the results from the 3 titrations?

Answer 29

1 - 19.9cm³ 
2 - 19.6cm³ 
3 - 19.5cm³ 

Question 30

The titre results should be within …. cm³ of each other for accuracy

Answer 30

0.1cm³ 

Question 31

What was the average titre?

Answer 31

19.6cm³ + 19.5cm³ divided by 2 = 19.55cm³ 

Question 32

What was the balanced equation?

Answer 32

2HCl + Na2CO3 –> 2NaCl + H2O + CO2

Question 33

Calculate moles/litre

Answer 33

V1 - 19.55cm³  V2 - 25cm³ 
M1 - x M2 - 0.1M
n1 - 2 n2 - 1
x = .256mol/l
Mr of HCl = 1 + 35.5 = 36.5
.256 x 36.5 = 9.344 g/l
Concentration = .256 mol/l or 9.344 g/l

Question 34

Why were only a few drops of indicator used?

Answer 34

Indicators are themselves either weak acids or weak bases and will therefore take part in the titration thereby affecting the overall result of the experiment. To minimise their taking part and have a significant effect on the result, the smallest amount possible is used.

Question 35

Explain the term ‘primary standard’ (L.C)

Answer 35

Can be dissolved to make up a solution of exact concentration, pure

Question 36

Describe how the student should have prepared 500cm³ of the 0.05M primary standard solution from a known mass of pure anhydrous sodium carbonate, supplied on a clock glass. (L.C)

Answer 36

• rinse from clock glass into beaker and dissolve
• Pour using funnel into 500cm³ volumetric flask and add rinsings of beaker
• read at eye level

Question 37

Describe how the liquid level in the burette was adjusted to the zero mark. (L.C)

Answer 37

Fill above mark and adjust with tap

Question 38

Why was a pipette filler used to fill the pipette with 25cm³ of the sodium carbonate solution? (L.C)

Answer 38

Safety / hygiene

Question 39

Name a suitable indicator for this titration. (L.C)

Answer 39

Methyl orange

Question 40

State the colour change observed at the end point. (L.C)

Answer 40

Pink/red

Question 41

Calculate, correct to two decimal places, the concentration of the hydrochloric acid solution in (i) moles per litre, and (ii) grams per litre. (L.C)

Answer 41

(i) 20.8 x M(HCl) divided by 2 = 25 x 0.05 divided by 1
M(HCl) = 0.12
(ii) 0.12 x 36.5 = 4.38