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A student investigating the water quality of stormwater in a drain near the school collected samples for testing in the school laboratory - HSC - SSCE Chemistry - Question 28 - 2011 - Paper 1
Question 28
A student investigating the water quality of stormwater in a drain near the school collected samples for testing in the school laboratory. The student conducted the ... show full transcript
Worked Solution & Example Answer:A student investigating the water quality of stormwater in a drain near the school collected samples for testing in the school laboratory - HSC - SSCE Chemistry - Question 28 - 2011 - Paper 1
Step 1
Hardness
Answer
The hardness test of water measures the concentration of calcium and magnesium ions present. The principle involved is the chemical reaction between these ions and a specific reagent, typically EDTA (ethylenediaminetetraacetic acid), which forms a complex with calcium and magnesium.
Procedure:
- Prepare a sample of stormwater in a clean beaker.
- Add a few drops of a buffer solution to maintain a stable pH during the titration.
- Titrate with a standard solution of EDTA until a color change occurs, which indicates that all calcium and magnesium ions have reacted.
- Record the volume of EDTA used to calculate the hardness of the water using the formula: ext{Hardness (mg/L)} = rac{ ext{Volume of EDTA (mL)} imes ext{EDTA concentration (mg/L)}}{ ext{Sample volume (mL)}}
Step 2
Biochemical oxygen demand
Answer
Biochemical Oxygen Demand (BOD) is a measure of the amount of dissolved oxygen required by aerobic microorganisms to decompose organic matter in water. The principle involves the bacterial decomposition of organic substances under controlled conditions over a certain time period.
Procedure:
- Collect a water sample in a BOD bottle, ensuring no air bubbles are trapped inside.
- Incubate the sample at 20°C for a period of 5 days in the dark to prevent photosynthesis.
- Measure the initial dissolved oxygen level just before incubation and the final level after 5 days.
- Calculate BOD using the formula: where DO represents dissolved oxygen levels.