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The concentration of sodium hypochlorite in swimming pool water can be determined by redox titration - Scottish Highers Chemistry - Question 12 - 2015
Question 12
The concentration of sodium hypochlorite in swimming pool water can be determined by redox titration. Step 1 A 100.0 cm³ sample from the swimming pool is first reac... show full transcript
Worked Solution & Example Answer:The concentration of sodium hypochlorite in swimming pool water can be determined by redox titration - Scottish Highers Chemistry - Question 12 - 2015
Step 1
Describe in detail how a burette should be prepared and set up, ready to begin the titration.
Answer
To prepare a burette for titration, follow these steps:
- Rinse the burette with distilled water to eliminate any impurities.
- Rinse the burette with the sodium thiosulfate solution several times, ensuring the solution coats the interior surface.
- Ensure that the burette is upright and securely clamped.
- Fill the burette with the thiosulfate solution just above the zero mark, then open the stopcock to allow excess solution to flow out, ensuring no air bubbles are present.
- Finally, make sure the liquid level is at the bottom of the meniscus to accurately measure the volume.
Step 2
Step 3
Calculate the concentration, in mol l⁻¹, of sodium hypochlorite in the swimming pool water, if an average volume of 12.4 cm³ of sodium thiosulfate was required.
Answer
To calculate the concentration of sodium hypochlorite:
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First determine the moles of sodium thiosulfate used:
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The stoichiometry from the equation shows that 1 mole of NaOCl reacts with 2 moles of thiosulfate:
ext{Moles of NaOCl} = rac{0.0000124}{2} = 0.0000062 ext{ mol}
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The concentration of sodium hypochlorite in 100 cm³ (0.1 l) of solution is:
ext{Concentration} = rac{ ext{Moles}}{ ext{Volume}} = rac{0.0000062 ext{ mol}}{0.1 ext{ l}} = 0.000062 ext{ mol l}^{-1}
Step 4
Calculate the volume of hypochlorite solution that will need to be added to an Olympic-sized swimming pool, capacity 2 500 000 litres, to raise the hypochlorite level from 1 ppm to 3 ppm.
Answer
To calculate the volume of hypochlorite solution needed:
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Determine the total volume of hypochlorite required to raise the level by 2 ppm (from 1 to 3 ppm):
Total volume = rac{2 ext{ ppm} imes 2,500,000 ext{ litres}}{1,000,000} = 5 ext{ litres}.
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Knowing that 400 cm³ raises the level by 1 ppm in 45,000 litres, calculate:
ext{Volume required} = 5 ext{ litres} imes rac{400 ext{ cm}^3}{1 ext{ ppm} imes 45,000 ext{ litres}} = rac{5 imes 400}{1 imes 45,000} imes 1,000,000 = ext{Volume needed in cm}³.
Step 5
Explain the difference in polarities of ammonia and trichloramine molecules.
Answer
Ammonia (NH₃) is a polar molecule due to the presence of a lone pair of electrons on the nitrogen atom, which creates an area of partial negative charge, while the hydrogen atoms have a partial positive charge. This results in an uneven distribution of electron density.
Trichloramine (NCl₃), on the other hand, is a non-polar molecule despite having polar N-Cl bonds. The symmetrical arrangement of the three chlorine atoms around the nitrogen atom cancels out the dipoles, resulting in an overall non-polar molecule.
Step 6
State what is meant by the term free radical.
Answer
A free radical is an atom or molecule that has unpaired electrons, making it highly reactive. Free radicals can initiate chain reactions in chemical processes and are often generated through various chemical reactions, such as the dissociation of chloramines under UV light.
Step 7