The concentration of sodium hypochlorite in swimming pool water can be determined by redox titration - Scottish Highers Chemistry - Question 12 - 2015

To calculate the concentration of sodium hypochlorite, we first find the moles of sodium thiosulfate used in the titration:

1. Calculate moles of Na2S2O3:
   Using the formula: \text{moles} = \text{concentration} \times \text{volume} \(in liters) $= 0.00100 \times 0.0124 = 0.0000124$

2. Determine moles of OCl⁻:
   Using the stoichiometric relationship (1:2 ratio from the reaction): $\text{moles of NaOCl} = \frac{0.0000124}{2} = 0.0000062$

3. Calculate concentration of NaOCl:
   Considering the total volume of the swimming pool sample (0.1 L): $\text{concentration} = \frac{0.0000062}{0.1} = 0.000062 \text{ mol L}^{-1}$

To calculate the volume needed to raise the hypochlorite level:

1. Calculate increase in hypochlorite:
   From 1 ppm to 3 ppm means an increase of 2 ppm. $2 \text{ ppm} = 2 \text{ mg of hypochlorite per litre of water}$

2. Total mass increase for 2,500,000 litres:
   $\text{total mass} = 2 \text{ mg/litre} \times 2,500,000 \text{ litres} = 5,000,000 \text{ mg} = 5,000 \text{ grams}$

3. Calculate volume of hypochlorite solution added:
   Using the total mass and the concentration of the hypochlorite solution (let's assume 400 cm³ raises by 1 ppm equating to 1 gram, hence 400 cm³ = 1 g): $\text{volume} = \frac{5,000 \text{ g}}{1 \text{ g/400 cm}^3} = 2,000,000 \text{ cm}^3 = 2000 \text{ litres}$

Ammonia (NH₃) is a polar molecule due to the significant electronegativity difference between nitrogen and hydrogen. The nitrogen atom has a partial negative charge, while the hydrogen atoms have partial positive charges, creating a dipole. In contrast, trichloramine (NCl₃) is non-polar overall. Although nitrogen has a partial negative charge, the presence of three chlorine atoms, which are more electronegative, leads to a balancing of charge distribution, resulting in a non-polar molecule.

A free radical is an atom or molecule that has unpaired valence electrons, making it highly reactive. Free radicals can participate in chemical reactions and originate from processes like UV treatment.

The type of step shown in the reaction between NH₂Cl and •Cl is known as a propagation step. In this step, a free radical reacts and generates another free radical, continuing the reaction chain.