Photo AI
A student does an experiment to determine the percentage by mass of sodium chlorate(I), NaClO, in a sample of bleach solution - AQA - A-Level Chemistry - Question 8 - 2020 - Paper 1
Question 8
A student does an experiment to determine the percentage by mass of sodium chlorate(I), NaClO, in a sample of bleach solution. Method: - Dilute a 10.0 cm³ sample o... show full transcript
Worked Solution & Example Answer:A student does an experiment to determine the percentage by mass of sodium chlorate(I), NaClO, in a sample of bleach solution - AQA - A-Level Chemistry - Question 8 - 2020 - Paper 1
Step 1
Use all the information given to calculate the percentage by mass of NaClO in the original bleach solution.
Answer
To calculate the mass of NaClO in the original bleach solution, we first determine the moles of sodium thiosulfate used.
-
Calculate moles of Na₂S₂O₃:
Moles of Na₂S₂O₃ =
\frac{33.50 , cm³}{1000} \times 0.00333 , mol , dm^{-3} = 0.000112 , mol -
Use the stoichiometry from the equations:
From the equations, we know that 2 moles of Na₂S₂O₃ react with 1 mole of I₃⁻. Thus: (0.000112 , mol , Na₂S₂O₃ \times \frac{1 , mol , I_3^-}{2 , mol , Na_2S_2O_3} = 0.000056 , mol , I_3^- )
-
Now, we find moles of NaClO:
I₃⁻ is formed from ClO⁻ according to the reactions provided. Therefore, the moles of NaClO in the original solution: ( 0.000056 , mol , ClO^- \text{ from I3^-} )
-
Convert moles of NaClO to grams:
The molar mass of NaClO is approximately 74.44 g/mol. Thus: (0.000056 , mol imes 74.44 , g/mol = 0.00417 , g)
-
Calculate the mass of bleach:
Volume of original bleach = 10.0 cm³, density = 1.20 g/cm³: ( ext{Mass of bleach} = 10.0 , cm³ \times 1.20 \frac{g}{cm³} = 12.0 , g)
-
Calculate percentage by mass of NaClO in bleach:
( \text{Percentage by mass} = \frac{0.00417 , g}{12.0 , g} \times 100 = 0.0346 , % )
Therefore, the percentage by mass of NaClO is approximately 0.035 % expressed to 3 significant figures = 0.0350%.
Step 2
What is the total percentage uncertainty in using the burette in this experiment?
Answer
The total uncertainty from the two readings and the end-point error in using the burette is ±0.15 cm³.
-
Calculate the percentage uncertainty for one reading:
- If we use a burette that measures to a total of 50 cm³, the percentage uncertainty for one reading is:
( \text{Percentage uncertainty} = \frac{0.15 , cm³}{50 , cm³} \times 100 = 0.30% )
-
Total uncertainty:
Since there are two readings, the total uncertainty is simply doubling the single reading uncertainty:
(\text{Total uncertainty} = 0.30% + 0.30% = 0.60% )
The total percentage uncertainty in using the burette is therefore 0.45%.