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 measure 50.0 cm³ of hard water, use a clean, dry 25 cm³ pipette twice to draw the liquid. First, rinse the pipette with distilled or deionized water followed by the hard water sample. Position the pipette vertically, and ensure the meniscus sits at eye level while reading the measurement accurately. Drain the water into a conical flask, allowing it to flow freely without blowing out. Adjust the pipette to exactly 50.0 cm³ and repeat the process to ensure accuracy.

The indicator used is Eriochrome Black T.

In the presence of M²⁺, the indicator appears wine red.

At the end point, the colour changes to blue.

The solution added before the titration is a buffer solution.

The buffer solution was added to ensure the pH of the reaction mixture remains constant, which is crucial for accurate titration results.

The average number of moles of edta used can be calculated using the formula:

Number of moles = Concentration (mol/L) × Volume (L)
= 0.010 mol/L × (9.3 cm³ / 1000) = 0.000093 moles.

Using the mole ratio from the balanced equation, the number of moles of M²⁺ can be found:
If 0.000093 moles of edta react with M²⁺, then the required amount for 50.0 cm³ is:
= 0.000093 moles.

Total hardness can be calculated by converting moles of M²⁺ to grams of CaCO₃:
Total hardness (mg/L) = (0.000093 moles) × (100.09 g/mol CaCO₃) = 9.30 mg/L.

To determine temporary hardness, boil a sample of the water. If it results in a significant precipitate (scum), the water contains temporary hardness, which is typically due to the presence of bicarbonates that can decompose upon heating.