Define a base according to (i) the Arrhenius theory, (ii) the Brønsted-Lowry theory - Leaving Cert Chemistry - Question b - 2013

According to the Brønsted-Lowry theory, a base is a proton (H⁺) acceptor, meaning it can accept hydrogen ions in a chemical reaction.

The conjugate acid of HPO₄²⁻ is H₂PO₄⁻.

The conjugate base of HPO₄²⁻ is PO₄³⁻.

First, calculate the concentration of NH₄OH in mol/L: $\text{moles of NH₄OH} = \frac{7.0 \text{ g}}{35 \text{ g/mol}} = 0.2 \text{ mol.}$

Thus, the concentration is 0.2 M.

Next, calculate the hydroxide ion concentration:

• For NH₄OH, the base dissociation constant is given, therefore: $K_b = [OH^-]^2 / [NH₄^+]$

Calculate [OH⁻]: $[OH^-] = \sqrt{K_b \times [NH₄OH]_{initial}} = \sqrt{(1.8 \times 10^{-5}) \times 0.2} = 1.9 \times 10^{-3} \text{ M}.$

Then, calculate pOH: $pOH = - \log(1.9 \times 10^{-3}) = 2.72.$

Finally, calculate pH using the relationship: $pH = 14 - pOH = 14 - 2.72 = 11.28.$

Thus, the pH of the solution is approximately 11.28.