According to Brønsted-Lowry theory, what is an acid? How does this theory distinguish between a strong and a weak acid? What is (i) the conjugate acid, (ii) the conjugate base, of HSO₄⁻? When carbon dioxide is pumped into still mineral water, it becomes sparkling water - Leaving Cert Chemistry - Question 7 - 2020

The distinction lies in the ability or tendency of the acid to donate protons. A strong acid completely dissociates in water, meaning it has a high degree of ionization and donates protons readily. In contrast, a weak acid only partially dissociates, resulting in a lower concentration of protons in solution.

(i) The conjugate acid of HSO₄⁻ is H₂SO₄, formed when HSO₄⁻ accepts a proton. (ii) The conjugate base of HSO₄⁻ is SO₄²⁻, formed when HSO₄⁻ donates a proton.

To test for sulfate ions, you could add barium chloride solution to the mineral water. If sulfate ions are present, a white precipitate of barium sulfate (BaSO₄) will form, indicating the presence of sulfate.

Given the pH of sparkling water is 5.6, we can first calculate the [H⁺] concentration:

First, we need to find the pOH of the source water: $ext{pOH} = 14 - ext{pH}\ = 14 - 7.2 = 6.8$ Then, using the pOH to find $ext{[OH}^-] = 10^{- ext{pOH}} = 10^{-6.8} = 1.58 imes 10^{-7} ext{ moles per litre}.$

Using the formula: $ext{pH} = - ext{log}K_a - ext{log}[ ext{HA}]$ We substitute the provided values to calculate the pH. $ext{pH} = - ext{log}(4.0 imes 10^{-4}) - ext{log}(1.5 imes 10^{-3})$

The dissociation of the indicator can be represented as:

When a few drops of the indicator are added to a strong base, the solution will shift towards the conjugate base form (A⁻), resulting in a color change to yellow. This occurs because the base reacts with the H⁺ ions, driving the equilibrium to the right.