1. The total hardness in a water supply was estimated by titrating 50.0 cm³ samples of the water with a standard solution of edta (ethylenediaminetetraacetic acid) - Leaving Cert Chemistry - Question 1 - 2018

To accurately measure 50.0 cm³ of hard water from a beaker into a conical flask:

1. Use a clean 25 cm³ pipette and rinse it with deionized water, then rinse it with a sample of the hard water.
2. Use the pipette to draw the hard water sample, making sure to fill the pipette just above the 50.0 cm³ mark.
3. Place your thumb over the top of the pipette to hold the liquid and bring it to eye level.
4. Slowly release the liquid into the conical flask, ensuring that the bottom of the meniscus is aligned with the 50.0 cm³ mark of the pipette.
5. Adjust the volume if necessary, ensuring not to drop any liquid outside the conical flask.

The indicator used is known as Erichrome Black T.

In the presence of M²⁺ ions, the colour observed with Erichrome Black T is wine/red.

At the end point of the titration, the colour changes to blue.

The solution added before the titration is typically a pH 10 buffer solution.

This buffer solution is added to ensure that the pH remains stable throughout the titration, which is crucial for the indicator to function properly.

To calculate the moles of edta used, use the formula: $ext{Number of moles} = ext{Concentration (M)} imes ext{Volume (L)}$ Here, the volume used is 9.3 cm³, converted to liters: $9.3 ext{ cm}^3 = 0.0093 ext{ L}$ Thus, $ext{moles of edta} = 0.010 imes 0.0093 = 9.3 imes 10^{-5} ext{ moles}$.

Given a 1:1 reaction ratio, the number of moles of M²⁺ is equal to the number of moles of edta used: $ext{M}^{2+} = 9.3 imes 10^{-5} ext{ moles}$.

The molar mass of CaCO₃ is approximately 100.09 g/mol. Thus, the mass of hardness can be calculated as follows: $ext{mass} = ext{moles} imes ext{molar mass} = 9.3 imes 10^{-5} imes 100.09 ext{ g/mol} \\ = 0.0093 ext{ g} = 9.3 ext{ mg}$.

To find p.p.m., use the formula: ext{p.p.m.} = rac{( ext{mass of CaCO}_3 ext{ in mg})}{ ext{volume of water in L}} imes 10^6 Substituting the values: ext{p.p.m.} = rac{9.3 ext{ mg}}{0.050 ext{ L}} imes 10^6 \\ = 186 ext{ p.p.m.}

To determine if the water supply contains temporary hardness, one could boil a sample of the water. If boiling removes the hardness and the water becomes soft, it indicates that the hardness was temporary (due to bicarbonate ions). Permanent hardness would not be affected by boiling.