7.1 Define an acid in terms of the Lowry-Brønsted theory - NSC Physical Sciences - Question 7 - 2016 - Paper 2

The conjugate base of H2CO3(aq) is HCO3⁻.

The balanced equation for the first step in the ionisation of carbonic acid is:

$H_2CO_3 (aq) \leftrightarrow HCO_3^- (aq) + H^+ (aq)$

To find the hydroxide ion concentration, we first calculate the hydrogen ion concentration using the pH value:

$pH = -\log[H^+]\Rightarrow [H^+] = 10^{-pH} = 10^{-3.4} \approx 3.98 \times 10^{-4} \, mol \, dm^{-3}$

Next, we use the relation between hydrogen ion concentration and hydroxide ion concentration at 25 °C:

$K_w = [H^+][OH^-] = 1.0 \times 10^{-14}$

Thus,

$[OH^-] = \frac{K_w}{[H^+]} = \frac{1.0 \times 10^{-14}}{3.98 \times 10^{-4}} \approx 2.51 \times 10^{-11} \, mol \, dm^{-3}$

A monoprotic acid is defined as an acid that can donate only one proton ($H^+$) per molecule when it ionizes in solution.

At the endpoint of the titration, the moles of sodium hydroxide ($NaOH$) and acid X are equal, so:

$n(NaOH) = c \times V = 1.0 \, mol\cdot dm^{-3} \times 0.0275 \, dm^3 = 0.0275 \, mol$

Since the ratio is 1:1:

$n(acid \, X) = n(NaOH) = 0.0275 \, mol$

Using the volume of acid X:

$c(acid \, X) = \frac{n}{V} = \frac{0.0275 \, mol}{0.025 \, dm^3} = 1.1 \, mol\cdot dm^{-3}$

Acid X is a strong acid because it ionizes completely in solution. The concentration of $H_3O^+$ ions in the sample of acid X is 2.4 × 10⁻⁴ mol·dm⁻³, which is lower than the concentration of 1.1 mol·dm⁻³ calculated in QUESTION 7.3.2. This indicates that the acid does not remain intact in solution, confirming that it is not weak.