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You are to determine the concentration of a solution of sodium thiosulfate, Na2S2O3 - CIE - A-Level Chemistry - Question 1 - 2014 - Paper 1
Question 1
You are to determine the concentration of a solution of sodium thiosulfate, Na2S2O3. To do this you will first produce a known amount of iodine by reacting iodate(I... show full transcript
Worked Solution & Example Answer:You are to determine the concentration of a solution of sodium thiosulfate, Na2S2O3 - CIE - A-Level Chemistry - Question 1 - 2014 - Paper 1
Step 1
Method
Answer
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Fill a burette with FB 1: Start by ensuring that your burette is clean and filled with the sodium thiosulfate solution (FB 1).
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Pipette 25.0 cm³ of FB 2 into the conical flask: Use a pipette accurately to measure 25.0 cm³ of the potassium iodate solution (FB 2), and transfer it into a conical flask.
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Add potassium iodide: Using a measuring cylinder, measure 10 cm³ of the aqueous potassium iodide (FB 4) and add it to the conical flask. At this point, a brown iodine solution will be produced.
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Add sulfuric acid: Pour in sulfuric acid (FB 3) to the conical flask until the iodine color disappears and the solution appears clear. This will ensure all iodine is in the solution.
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Titrate with FB 1: Gradually add sodium thiosulfate from the burette to the conical flask until a blue-black color appears. This indicates that a titration reaction is occurring.
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Record the rough titre: Note the volume of sodium thiosulfate used in the rough titration.
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Perform accurate titrations: Repeat the titration process several times to obtain consistent results, ensuring recorded titers reflect accuracy.
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Document results: Record all volume readings of FB 1 added during each titration in a table for observation, ensuring accuracy.
Step 2
From your accurate titration results, obtain a suitable value to be used in your calculations.
Answer
To calculate a suitable value from your accurate titration results, average two or more accurate titrate readings with a total spread of no more than 0.2 cm³. Ensure that these selected titration values are within 0.2 cm³ of each other for accurate averaging. The average should be recorded to two decimal places.
Step 3
(i) Calculate the number of moles of KIO3 present in 25.0 cm³ of FB 2.
Answer
The moles of KIO3 can be calculated using the formula:
where:
- is the concentration of KIO3 in g dm (given as 3.60 g dm)
- is the volume in cm³ (25.0 cm³)
Inserting the values, we get:
Thus, moles of KIO3 = 0.090 mol.
Step 4
(ii) Use these equations to calculate the number of moles of thiosulfate present in the volume of FB 1 you calculated in (b).
Answer
Using the stoichiometry of the reaction, where 1 mole of I2 reacts with 2 moles of S2O32-. The moles of iodine produced can be determined and subsequently, the moles of thiosulfate can be calculated. If the number of moles of iodine is known from part (i), the moles of S2O32- can be determined using:
Substituting the previously calculated value of I2 moles will give the final count of moles of thiosulfate.
Step 5
(iii) Calculate the concentration, in mol dm-3, of sodium thiosulfate in FB 1.
Answer
The concentration of sodium thiosulfate can be calculated by the formula:
Where:
- is the number of moles of S2O32- calculated in part (ii)
- is the volume of FB 1 used in dm³ (which will need to be converted from cm³ to dm³ by dividing by 1000).
This gives the final concentration in mol dm.