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The concentration of sodium hypochlorite in swimming pool water can be determined by redox titration - Scottish Highers Chemistry - Question 12 - 2023
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 - 2023
Step 1
Step 2
(ii) 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, we first determine the moles of sodium thiosulfate used:
- Moles of sodium thiosulfate = Concentration × Volume = 0.00100 mol L⁻¹ × 0.0124 L = 0.0000124 moles.
According to the redox reaction:
The stoichiometry indicates 1 mole of NaOCl reacts with 2 moles of I⁻. Therefore, the moles of sodium hypochlorite can be calculated as:
- Moles of NaOCl = 0.0000124 moles / 2 = 0.0000062 moles.
Now, we convert this to concentration:
- Concentration of NaOCl = Moles / Volume of sample in L = 0.0000062 moles / 0.1 L = 0.000062 mol L⁻¹.
Step 3
(b) 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
The difference in hypochlorite level is 3 ppm - 1 ppm = 2 ppm.
To calculate the volume of hypochlorite solution needed:
1 ppm in 2 500 000 litres = 2 500 000 g water, which corresponds to 2 500 g.
To raise by 2 ppm:
2 g hypochlorite × 2 500 000 litres = 5000 g of hypochlorite needed.
The concentration of the commercial hypochlorite solution is:
- 400 cm³ raises by 1 ppm in 45,000 litres, thus: 1 ppm corresponds to 400 cm³ hypochlorite in 45.000 litres.
Using this ratio:
- Volume required = (5000 g/ 1 g) × 400 cm³ / 45 000 L = approx. 44.44 L.
Step 4
(i) Explain the difference in polarities of ammonia and trichloramine.
Answer
Ammonia (NH₃) is a polar molecule due to its trigonal pyramidal shape and the electronegativity difference between nitrogen and hydrogen.
the nitrogen atom pulls the electron density towards itself, creating a dipole where the nitrogen end is partially negative and the hydrogen ends are partially positive.
Trichloramine (NCl₃), on the other hand, has a similar trigonal pyramidal structure, but the three chlorine atoms are much more electronegative than nitrogen.
Thus, the dipole moments due to N-Cl bonds cancel out, making trichloramine a non-polar molecule.
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Step 6