The total hardness values of a water supply before and after it had been passed through a laboratory deioniser were compared - Leaving Cert Chemistry - Question 1 - 2022

Total hardness refers to the concentration of dissolved calcium and magnesium salts in water. It is typically expressed as the equivalent concentration of calcium carbonate (CaCO₃) and represents the cumulative effect of these ions on the overall water quality.

A suitable indicator for this titration is Eriochrome Black T.

The colour change observed at the end point is from wine red (in the presence of Ca²⁺ and Mg²⁺) to blue, indicating the formation of the M²⁺-edta complex.

The buffer solution maintains a pH around 10 in the mixture in the conical flask.

Maintaining this pH is important because it ensures that the metal ions react properly with edta and that the indicator functions correctly, preventing erroneous results in the titration process.

To prepare the burette, follow these steps:

1. Clamp the burette vertically.
2. Fill the burette below zero, then fill to zero using a dropper.
3. Ensure there are no bubbles in the nozzle and clean the dry funnel.
4. Adjust the burette to the correct initial mark at eye level to avoid parallax errors.

To calculate the number of moles of edta, use the formula:

$ext{Number of moles} = ext{Concentration} imes ext{Volume}$

Therefore, we have:

ext{Number of moles} = 0.010 imes rac{9.2}{1000} = 0.000092 moles.

Using the stoichiometry of the reaction, 1 mole of edta reacts with 1 mole of M²⁺:

Thus, the number of moles of M²⁺ is:

Given that 0.00368 moles of M²⁺ is equivalent to the moles removed from 1 litre:

$(0.000092 imes 10) = 0.00368 ext{ moles of M²⁺ removed from 1 litre}$.

To calculate total hardness, convert moles of M²⁺ into grams of CaCO₃:

Total mass of CaCO₃:

$ext{Molar mass of CaCO₃} = 100 ext{ g/mol}.$

Thus,

$ext{Total hardness} = 0.00368 ext{ moles} imes 100 ext{ g/mol} = 0.368 ext{ g of CaCO₃}$, which is equivalent to 370 p.p.m. CaCO₃ in 1 litre.