The quality of water samples from swimming-pools A, B, C and D was assessed - Leaving Cert Chemistry - Question 3 - 2015

Given that the dissolved solids in sample B is 1400 p.p.m., we can calculate the mass as follows:

First, convert 250 cm³ to liters: $250 ext{ cm}^3 = 0.250 ext{ L}$

Now, using the conversion factor:

$\text{Mass (g)} = \text{Concentration (p.p.m.)} \times \text{Volume (L)} \times \frac{1 ext{ g}}{1000 ext{ p.p.m.}}$

Substituting the values:

$\text{Mass} = 1400 imes 0.250 \times \frac{1}{1000} = 0.35 ext{ g}$

The solution titrated against sample C to measure its total hardness is EDTA (ethylenediaminetetraacetic acid). EDTA forms complexes with calcium and magnesium ions, which are responsible for hardness, allowing for their quantification.

The second result was significantly lower because boiling the sample likely removed temporary hardness. When water is boiled, bicarbonate ions (HCO₃⁻) convert to carbon dioxide (CO₂) which escapes as gas, thereby reducing the total hardness measured in the second test.

DPD (diethyl-p-phenylenediamine) or KI (potassium iodide) is used to detect free chlorine in water samples. DPD reacts with free chlorine to produce a pink color.

In using a comparator, a sample of the water is compared against color standard solutions of known free chlorine concentrations. The intensity of the pink color in the test sample can then be matched to the standards for quantification.

In a colorimeter, the absorbance of the pink solution is measured, allowing the chlorine concentration to be determined based on a calibration curve created from known concentrations.

Pools A and D were considered unsuitable for swimming due to high levels of suspended solids and low levels of free chlorine, respectively. For pool A, the concentration of suspended solids (500 p.p.m.) exceeds acceptable limits, and for pool D, the free chlorine level is too high (2.1 p.p.m.), potentially causing harm to swimmers.

Swimmers in pool A may experience skin irritation or respiratory issues due to the high concentration of suspended solids, which can harbor bacteria and other contaminants, leading to health risks.

In pool D, the elevated free chlorine level (2.1 p.p.m.) can cause discomfort, such as skin irritation, eye irritation, and respiratory issues. Furthermore, high chlorine levels can be harmful in the long term.