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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: 1. Dilute a 10.0 cm³ sample ... 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 the information given to calculate the percentage by mass of NaClO in the original bleach solution.
Answer
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Calculate moles of Na₂S₂O₃ used:
r(Na₂S₂O₃) = rac{33.50 ext{ cm}^3 imes 1.00}{1000} imes 0.00333 ext{ mol} ext{ cm}^{-3} = 0.03335 ext{ mol}
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From the balanced equation (reaction 2):
r(I₃⁻) = rac{1}{2} r(Na₂S₂O₃)
r(I₃⁻) = rac{1}{2} imes 0.03335 ext{ mol} = 0.016675 ext{ mol}
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Calculate moles of NaClO using the first equation:
r(NaClO) = 0.00670 ext{ mol}
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Mass of bleach:
m(NaClO) = r(NaClO) imes 74.44 ext{ g/mol} = 0.00670 ext{ mol} imes 74.44 ext{ g/mol} = 0.4999 ext{ g}
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Mass of bleach solution:
m(bleach) = 100 imes 1.20 ext{ g} = 120 ext{ g}
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Calculate the percentage by mass:
ext{% NaClO} = rac{0.4999 ext{ g}}{120 ext{ g}} imes 100 = 0.41658 ext{ %}
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Round to 3 significant figures:
ext{% NaClO} = 0.417 ext{ %}
Step 2
What is the total percentage uncertainty in using the burette in this experiment?
Answer
The total uncertainty is calculated as follows:
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Total uncertainty = ± 0.15 cm³
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Calculate the percentage uncertainty:
ext{Percentage uncertainty} = rac{0.15 ext{ cm}³}{25.0 ext{ cm}³} imes 100 ext{ %} = 0.6 ext{ %}
Therefore, the correct box to tick is:
- 0.45%