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Chemistry Question HELP!!!

Someone please please help me with this question! It would be most appreciated!! This is an AQA A-level Chemistry question that contains maximum 7 marks.
I’ve been stuck on this question for a while now.

A student does an experiment to determine the percentage by mass of sodium chlorate(1), NaCIO, in a sample of bleach solution.
Method: . Dilute a 10.0 cm sample of bleach solution to 100 cm with distilled water. Transfer 25.0 cm3 of the diluted bleach solution to a conical flask and acidify using sulfuric acid. . Add excess potassium iodide to the conical flask to form a brown solution containing I2(aq) . Add 0. 100 mol dm- sodium thiosulfate solution (Na2S203) to the conical flask from a burette until the brown solution containing I2(aq) becomes a colourless solution containing I-(aq).

The student uses 33.50 cm of sodium thiosulfate solution. The density of the original bleach solution is 1.20 g cm-3.

The equations for the reactions in this experiment are
CIO-(aq) + 2H+(aq) + 2I-(aq) -> Cl-(aq) + H20(l) + I2(aq)
2S2032-(aq) + I2(aq) -> 2I-(aq) + S4062-(aq)

Use all the information given to calculate the percentage by mass of NaCIO in the original bleach solution.

@CHEMISTRY @charco
(edited 2 years ago)
Original post by Knav
Someone please please help me with this question! It would be most appreciated!! This is an AQA A-level Chemistry question that contains maximum 7 marks.
I’ve been stuck on this question for a while now.

A student does an experiment to determine the percentage by mass of sodium chlorate(1), NaCIO, in a sample of bleach solution.
Method: . Dilute a 10.0 cm sample of bleach solution to 100 cm with distilled water. Transfer 25.0 cm3 of the diluted bleach solution to a conical flask and acidify using sulfuric acid. . Add excess potassium iodide to the conical flask to form a brown solution containing I2(aq) . Add 0. 100 mol dm- sodium thiosulfate solution (Na2S203) to the conical flask from a burette until the brown solution containing I2(aq) becomes a colourless solution containing I-(aq).

The student uses 33.50 cm of sodium thiosulfate solution. The density of the original bleach solution is 1.20 g cm-3.

The equations for the reactions in this experiment are
CIO-(aq) + 2H+(aq) + 2I-(aq) -> Cl-(aq) + H20(l) + I2(aq)
2S2032-(aq) + I2(aq) -> 2I-(aq) + S4062-(aq)

Use all the information given to calculate the percentage by mass of NaCIO in the original bleach solution.

@CHEMISTRY @charco

ngl i have no clue but replying to try and boost, good luck x
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Knav
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Original post by medicnia20
ngl i have no clue but replying to try and boost, good luck x


Thank you 🤣🤣, people on student room are the real MVP
Original post by Knav
Someone please please help me with this question! It would be most appreciated!! This is an AQA A-level Chemistry question that contains maximum 7 marks.
I’ve been stuck on this question for a while now.

A student does an experiment to determine the percentage by mass of sodium chlorate(I), NaCIO, in a sample of bleach solution.
Method: . Dilute a 10.0 cm sample of bleach solution to 100 cm with distilled water. Transfer 25.0 cm3 of the diluted bleach solution to a conical flask and acidify using sulfuric acid. . Add excess potassium iodide to the conical flask to form a brown solution containing I2(aq) . Add 0. 100 mol dm- sodium thiosulfate solution (Na2S203) to the conical flask from a burette until the brown solution containing I2(aq) becomes a colourless solution containing I-(aq).

The student uses 33.50 cm of sodium thiosulfate solution. The density of the original bleach solution is 1.20 g cm-3.

The equations for the reactions in this experiment are
CIO-(aq) + 2H+(aq) + 2I-(aq) -> Cl-(aq) + H20(l) + I2(aq)
2S2032-(aq) + I2(aq) -> 2I-(aq) + S4062-(aq)

Use all the information given to calculate the percentage by mass of NaCIO in the original bleach solution.

@CHEMISTRY @charco

Dilute a 10.0 cm3 sample of bleach solution to 100 cm with distilled water.

This is a 10:1 dilution. You need this right at the end.

Transfer 25.0 cm3 of the diluted bleach solution to a conical flask and acidify using sulfuric acid. Add excess potassium iodide to the conical flask to form a brown solution containing I2(aq) .

Note that you have used EXCESS KI(aq). This ensures that all of the ClO- ions react. Now look at the first equation:

CIO-(aq) + 2H+(aq) + 2I-(aq) -> Cl-(aq) + H20(l) + I2(aq)

What is the relationship between mol of chlorate(I) reacted and mol of iodine formed?


Add 0. 100 mol dm- sodium thiosulfate solution (Na2S2O3) to the conical flask from a burette until the brown solution containing I2(aq) becomes a colourless solution containing I-(aq).

Now you need the second equation.

2S2O32-(aq) + I2(aq) -> 2I-(aq) + S4O62-(aq)

What is the relationship between mol of thiosulfate reacted and mol of iodine?

The student uses 33.50 cm3 of sodium thiosulfate solution. The density of the original bleach solution is 1.20 g cm-3.


You now need to work out the mol of sodium thiosulfate using the volume and density data.

Imagine that you have 1000cm3. The mass will be 1200g. If this were pure water you would expect a mass of 1000g. Hence there is an excess mass of 200 g

Therefore there are 200g of Na2S2O3 dissolved in 1dm3 of solution.

mass/relative mass gives moles.
Moles/ volume gives concentration.

You can now work out the mol of thiosulfate in 33.5cm3.

So you can work out the mol of iodine.

And hence the mol of chlorate(I) in the sample taken.

Scale up to get mol chlorate(I) in the whole sample.

Now look at the original dilution.

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