Titration - standardisation of HCl solution using a standard solution of anhydrous Na₂CO₃ Flashcards
Strong acid - strong base titration
Apparatus needed
(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
Materials needed
- .1 mol/l Na₂CO₃
- HCl solution
- Methyl orange indicator
What is the first step?
Pipette, burette and conical flask were washed with deionised water
What was the burette washed with?
Some of the HCl solution
Why was the burette washed with some of the HCl solution?
To prevent dilution of the solution
What happens to the burette after it has been washed with some of the HCl solution?
It was clamped vertically using the retort stand
What happens after the burette gets clamped?
Using a funnel the burette was then filled with the HCl solution.
Why was the tap opened briefly when the burette was filled with the HCl solution?
To allow the bit below the tap to fill.
What happened after the funnel was used?
Funnel was removed and the solution was allowed to run off until the bottom of the meniscus was on the zero graduation mark.
What happened after the funnel was removed?
Some of the sodium carbonate solution was poured into a clean dry beaker
What happened after some of the sodium carbonate solution was poured into a clean dry beaker?
The pipette was rinsed out with some of this solution.
Why was the pipette rinsed out with some of the solution already made?
To prevent a change in concentration.
What happened after the pipette was rinsed out with some solution?
25cm³ of the sodium carbonate solution were pipetted into a clean conical flask
What was added to the conical flask after the sodium carbonate solution?
3 drops of methyl orange indicator.
What colour did the solution become after the methyl orange indicator was added?
Yellow (in alkaline solution)
What happened after the indicator was added?
The conical flask was placed on a white tile
Why was the conical flask placed on the white tile after the indicator was added?
So an accurate colour change would be seen to indicate the end-point.
What happened after the conical flask was placed on the white tile?
The tap on the burette was opened and some of the HCl solution was added to the conical flask.
What happened after some of the HCl solution was added to the conical flask?
The conical flask was swirled continuously
What happened during the titration?
It was washed down with deionised water
Why was the conical flask washed down with deionised water during the titration?
To ensure that all the HCl molecules react with the sodium carbonate in the conical flask.
The titration was continued until the solution turned…..
Pink
What was noted on the burette after the titration ended?
The titre was noted
What was the first titration called?
The rough titration result
Why is the rough titration important
So that the subsequent (next) titres can be more accurate.
Which titration is ignored when calculating the titration?
The first one
How many more times was the titration repeated?
2
What happened in the next two titrations?
The HCl solution was added drop by drop as the end-point neared.
What were the results from the 3 titrations?
1 - 19.9cm³
2 - 19.6cm³
3 - 19.5cm³
The titre results should be within …. cm³ of each other for accuracy
0.1cm³
What was the average titre?
19.6cm³ + 19.5cm³ divided by 2 = 19.55cm³
What was the balanced equation?
2HCl + Na2CO3 –> 2NaCl + H2O + CO2
Calculate moles/litre
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
Why were only a few drops of indicator used?
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.
Explain the term ‘primary standard’ (L.C)
Can be dissolved to make up a solution of exact concentration, pure
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)
- rinse from clock glass into beaker and dissolve
- Pour using funnel into 500cm³ volumetric flask and add rinsings of beaker
- read at eye level
Describe how the liquid level in the burette was adjusted to the zero mark. (L.C)
Fill above mark and adjust with tap
Why was a pipette filler used to fill the pipette with 25cm³ of the sodium carbonate solution? (L.C)
Safety / hygiene
Name a suitable indicator for this titration. (L.C)
Methyl orange
State the colour change observed at the end point. (L.C)
Pink/red
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)
(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
