The chlorine demand of a water sample was found to be 0.2 mg/litre. The amount of bleaching powder containing 30% available chlorine to be added to treat one litre of such a water sample is
0.67 mg
Chlorine Demand: Calculating Bleaching Powder Dosage
This solution explains how to determine the correct amount of bleaching powder required to treat water, based on its chlorine demand and the strength of the bleaching powder.
Understanding Chlorine Demand
The chlorine demand of a water sample is the amount of chlorine that is consumed by impurities (like organic matter, bacteria, etc.) present in the water during the process of chlorination. It's the quantity of chlorine needed to satisfy these reactions before the water becomes chlorinated (disinfected). In this problem, the chlorine demand is given as 0.2 mg/litre.
Understanding Bleaching Powder Strength
Bleaching powder is commonly used for water disinfection. However, it doesn't contain 100% pure chlorine. The question states that the bleaching powder used contains 30% available chlorine. This means that out of 100 mg of bleaching powder, only 30 mg is effective chlorine that can react with impurities or disinfect the water.
Calculation Steps
We need to find the mass of bleaching powder required to provide 0.2 mg of actual chlorine per litre of water.
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Identify the required amount of chlorine: The chlorine demand is 0.2 mg/litre. This is the amount of *available chlorine* needed.
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Identify the strength of the bleaching powder: The bleaching powder has 30% available chlorine.
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Set up the equation: Let '$m$' be the mass of bleaching powder (in mg) required per litre of water. The amount of available chlorine provided by '$m$' mg of bleaching powder is $30\%$ of $m$, which can be written as $0.30 \times m$.
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Equate available chlorine to chlorine demand: The available chlorine provided must equal the chlorine demand.
$$ 0.30 \times m = 0.2 \, \text{mg/litre} $$
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Solve for '$m$':
$$ m = \frac{0.2 \, \text{mg/litre}}{0.30} $$
$$ m = \frac{2}{3} \, \text{mg/litre} $$
$$ m \approx 0.6667 \, \text{mg/litre} $$
Result Interpretation
The calculation shows that approximately 0.67 mg of bleaching powder is needed to treat one litre of water to meet the specified chlorine demand.
Matching with Options
Comparing the calculated value with the given options:
- Option 1: 0.06 mg
- Option 2: 0.14 mg
- Option 3: 1.33 mg
- Option 4: 0.67 mg
The calculated value of approximately 0.67 mg matches Option 4.
Important Questions from Disinfection
The dose of chlorination, beyond which any further addition of chlorine will appear as free residual chlorine, is known as:
A city that has a supply of 15000 cubic meters of water per day is treated with a chlorine dosage of 0.5 ppm. For this purpose, the requirement of 25% bleaching powder per day would be.
Which one of the following is used to kill the pathogens?
An experiment shows that a concentration of 0.1 g/m3 of free available chlorine yield a 99% kill of bacteria in 8 minutes. Calculate disinfection rate constant (1 /min)? Assume that Chick's Law and Watson's Law hold with n = 1.
Order 4 disinfectants in increasing order of their disinfection power?
