Photo AI
The quality of water samples from swimming-pools A, B, C and D was assessed - Leaving Cert Chemistry - Question 3 - 2015
Question 3
The quality of water samples from swimming-pools A, B, C and D was assessed. The results of the tests carried out are summarised in the table. Sample / Pool A B... show full transcript
Worked Solution & Example Answer:The quality of water samples from swimming-pools A, B, C and D was assessed - Leaving Cert Chemistry - Question 3 - 2015
Step 1
Describe how the concentration of suspended solids in sample A could have been measured.
Answer
To measure the concentration of suspended solids in sample A, a known volume of the water sample should be passed through a pre-weighed filter paper. The filter paper must be dry and free of any solids at the start. After filtration, the filter paper along with the retained solids is dried and reweighed. The difference in mass before and after filtration gives the mass of the suspended solids, which can then be expressed in parts per million (p.p.m.) by relating the mass of solids to the original volume of the sample.
Step 2
Calculate the mass in grams of dissolved solids in 250 cm<sup>3</sup> of sample B.
Answer
To calculate the mass of dissolved solids in 250 cm<sup>3</sup> of sample B:
Using the concentration of dissolved solids in sample B, which is 1400 p.p.m., the mass can be calculated using the formula:
ext{Mass (g)} = rac{ ext{Concentration (p.p.m.)} imes ext{Volume (L)}}{1,000,000}
Thus:
ext{Mass} = rac{1400 imes 0.250}{1,000,000} = 0.35 ext{ g}.
Step 3
What solution was titrated against a known volume of sample C to measure its total hardness?
Answer
The total hardness of sample C was measured by titrating it with EDTA (ethylenediaminetetraacetic acid). This reagent forms complexes with the calcium and magnesium ions in the sample, effectively allowing the quantification of hardness in terms of p.p.m. CaCO<sub>3</sub>.
Step 4
Suggest a reason why the second result was significantly lower than the first.
Answer
The second result was significantly lower because boiling sample C removes temporary hardness by precipitating carbonate ions. The boiling process allows calcium bicarbonate to convert to insoluble calcium carbonate, which is then removed through filtration. Consequently, fewer hardness ions remain, resulting in a lower measured total hardness.
Step 5
Identify a reagent (or reagents) used to detect free chlorine in a water sample. Describe how a comparator or a colorimeter could have been used to estimate the concentration of free chlorine in sample D.
Answer
The reagent commonly used to detect free chlorine is DPD (diethyl-p-phenylenediamine). In a colorimetric analysis, a sample of water is treated with DPD, which forms a pink compound in the presence of free chlorine.
To estimate the concentration using a comparator or colorimeter:
- Prepare standard solutions of known chlorine concentration between 0 to 5 p.p.m.
- Add DPD to both the water sample D and the standard solutions.
- Compare the color intensity of sample D with that of the standards or measure its absorbance using a colorimeter. The intensity of the pink color corresponds to the concentration of free chlorine in sample D.
Step 6
Which of the test results led to pools A and D being considered unsuitable for swimming?
Answer
Pools A and D were considered unsuitable for swimming due to their high levels of suspended solids and high hardness. Sample A had a suspended solids level of 350 p.p.m. and a hardness of 1280 p.p.m. CaCO<sub>3</sub>. Sample D showed even higher total hardness of 212 p.p.m. CaCO<sub>3</sub> and a significant level of free chlorine, which can irritate skin.
(i) In pool A, the high level of suspended solids can lead to cloudy water, making it difficult to see hazards and compromising safety.
(ii) In pool D, the elevated free chlorine concentration may cause skin irritations or respiratory issues for swimmers.