What is the Arrhenius definition of a base? Why is NH₃ considered to be a base according to Brønsted-Lowry theory? (6) Calculate the pH value of (i) a 0.50 M solution of hydrochloric acid, (ii) a 0.50 M solution of ethanoic acid - Leaving Cert Chemistry - Question b - 2018

According to Brønsted-Lowry theory, NH₃ (ammonia) is considered a base because it accepts a proton (H⁺). This characteristic defines it as a proton acceptor, which is the central aspect of Brønsted-Lowry acid-base theory.

For hydrochloric acid (HCl), which is a strong acid, the pH can be calculated using the following equation:

$pH = - ext{log}[H^+]$

Since HCl dissociates completely in solution,

$[H^+] = 0.50 ext{ M}$
So,

To calculate the pH of a 0.50 M solution of ethanoic acid (CH₃COOH), we use the acid dissociation constant (Kₐ):

K_a = rac{[H^+][A^-]}{[HA]}

Here, let the concentration of dissociated H⁺ be denoted as x:
Following the expression:

yielding the equation:

K_a = 1.8 imes 10^{-5} = rac{x*x}{0.50 - x}
For simplicity, we assume x is very small compared to 0.50, hence:

The acid solution that was neutralised can be identified from its pH curve. As sodium hydroxide (NaOH) is added, a steep rise in pH indicates that a strong acid was neutralised. If the pH rapidly increased before reaching a neutral point, it suggests that hydrochloric acid (HCl) has been neutralised.

1. The rapid increase in pH at the equivalence point demonstrates the strong acidic nature of HCl being neutralised by NaOH.
2. The pH remains relatively stable at 7 after neutralisation, indicating a complete reaction between the acid and base.

The essential property of an indicator used in this titration is that it must change color at a pH close to 7, which is the expected endpoint for strong acid and strong base titrations. Thus, the indicator should have a transition range (pH) that coincides with the steep vertical region of the pH curve.