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A number of tests were carried out on a sample of swimming pool water to test its quality - Leaving Cert Chemistry - Question 3 - 2006
Question 3
A number of tests were carried out on a sample of swimming pool water to test its quality. (a) A colorimetric experiment was used to estimate the concentration of f... show full transcript
Worked Solution & Example Answer:A number of tests were carried out on a sample of swimming pool water to test its quality - Leaving Cert Chemistry - Question 3 - 2006
Step 1
What is the general principle of all colorimetric experiments?
Answer
The general principle of colorimetric experiments is based on the relationship between the intensity (depth of color) of a solution and its concentration. Specifically, the absorbance or transmittance of light through a colored solution varies proportionally to the concentration of the absorbing species in the solution, according to Beer-Lambert Law. Thus, as the concentration increases, the depth of color becomes more intense.
Step 2
Identify a suitable reagent to test for free chlorine in swimming pool water and state the colour which develops when this reagent reacts with free chlorine.
Answer
A suitable reagent to test for free chlorine in swimming pool water is acidified potassium iodide (KI/I2). When free chlorine reacts with this reagent, a red-brown color develops due to the formation of iodine.
Step 3
Describe briefly how you would estimate the concentration of free chlorine in a sample using either a comparator or a colorimeter.
Answer
To estimate the concentration of free chlorine using a comparator:
- Prepare a standard colorimetric solution of known concentration of free chlorine.
- Add the water sample to a separate vessel in the comparator.
- Compare the color of the sample to the standard solutions to visually estimate concentration.
Using a colorimeter:
- Calibrate the colorimeter using standard solutions.
- Place the sample in the colorimeter after proper treatment.
- Take transmittance or absorbance readings and use a calibration curve to find the concentration.
Step 4
Give the name or formula of a free chlorine species in the swimming pool water. Give a reason why the concentration of free chlorine in treated drinking water is usually between 0.2 - 0.5 ppm, whereas in swimming pool water it should be between 1 - 5 ppm.
Answer
A common free chlorine species present in swimming pool water is hypochlorous acid (HOCl). The concentration of free chlorine in treated drinking water is maintained lower (0.2 - 0.5 ppm) primarily for safety, as high levels can be harmful to human health. In contrast, swimming pool water requires higher levels (1 - 5 ppm) to effectively disinfect the water and eliminate pathogens, given the higher volume of water and the potential for contamination from pool users.
Step 5
When 1200 cm³ of swimming pool water was filtered, the mass of the filter paper, upon drying, had increased by 0.78 g. When 250 cm³ of the filtered water was evaporated to dryness the mass of the residue obtained was 0.32 g. Calculate the concentration in p.p.m. of suspended solids.
Answer
To calculate the concentration of suspended solids in p.p.m.:
- The mass of the residue from evaporating 250 cm³ of filtered water is 0.32 g.
- Convert the volume of filtered water to liters: 250 cm³ = 0.25 L.
- Concentration of suspended solids = (mass of residue / volume of sample) × 1000.
Thus,
Step 6
Calculate the concentration in p.p.m. of dissolved solids.
Answer
To calculate the concentration of dissolved solids:
- The mass of residue from the 1200 cm³ sample is 0.78 g.
- Convert 1200 cm³ to liters: 1200 cm³ = 1.2 L.
- Concentration of dissolved solids = (mass of solids / volume of sample) × 1000.
Thus,