The Swimming Pool Chlorine Reading
A school pool holds 50,000 L of water. The caretaker measures the free chlorine concentration as 0.4 mg/L, below the recommended minimum of 1.0 mg/L. She adds 500 g of calcium hypochlorite powder labelled 65% available chlorine by mass. The powder dissolves completely. Free chlorine is consumed by sunlight and by swimmers at roughly 0.15 mg/L per hour during a busy afternoon. Two hours later a class of 30 students wants to swim, and the caretaker must decide whether the reading will still be above 1.0 mg/L.
1. Calculate the free chlorine concentration immediately after the powder dissolves, and state whether it reaches 1.0 mg/L.
Read the worked answer
Available chlorine added = 500 g x 0.65 = 325 g = 325,000 mg. Concentration added = 325,000 mg / 50,000 L = 6.5 mg/L. Total = 0.4 + 6.5 = 6.9 mg/L. This is far above 1.0 mg/L, so the minimum is met with a large margin.
2. Predict the concentration after two busy hours and decide whether the class should swim.
Read the worked answer
Loss = 0.15 mg/L per hour x 2 h = 0.3 mg/L. Predicted = 6.9 - 0.3 = 6.6 mg/L. This is still well above 1.0 mg/L, so the class can swim. However, 6.6 mg/L is much higher than the recommended range for comfort (often around 1-3 mg/L), so the caretaker should consider whether the dose was excessive and whether the pool should be partly diluted or left to decay before use.