(a) Define (i) an acid, (ii) a base according to the Brønsted-Lowry theory - Leaving Cert Chemistry - Question 8 - 2003

In the context of the Brønsted-Lowry theory, a base is defined as a proton acceptor. This means that a base is a substance that can accept a hydrogen ion (H⁺) from an acid during a chemical reaction.

In the given equilibrium:

$H_2S + OH^- \rightleftharpoons HS^- + H_2O$ The acid is $H_2S$ (as it donates a proton), and the conjugate acid is $HS^-$ (the species formed after the acid donates a proton).

pH is defined as the negative logarithm (base 10) of the hydrogen ion concentration

$pH = - ext{log}_{10}[H^+]$ This measures the acidity or basicity of a solution.

Given that the concentration of acetic acid is 6% (w/v), we can convert this to molarity.

1. Calculation of molarity:

   • 6 g of acetic acid in 100 mL solution means:
   • Molar mass of acetic acid (C₂H₄O₂) = 60 g/mol.
   • Molarity (M) = rac{6 ext{ g}}{60 ext{ g/mol}} imes rac{1000 ext{ mL}}{100 ext{ mL}} = 1 ext{ mol/L}

2. The dissociation constant, $K_a = 1.8 imes 10^{-5}$, can be used to find the concentration of hydrogen ions. We use the approximation:

   • $K_a = [H^+][A^-]/[HA]$ Assuming $[H^+] = [A^-]$ and $[HA] ext{ is approximately } 1 ext{ mol/L}$, we get:
   • $1.8 imes 10^{-5} = [H^+]^2 / (1)$
   • $[H^+] = \sqrt{1.8 imes 10^{-5}} ext{ which gives approximately } 0.00424 ext{ mol/L}.$

3. Finally, we can calculate pH:

   • $pH = - ext{log}_{10}(0.00424) ext{ which results in approximately } 2.37.$