Chemistry 9701 · AS & A Level · Equilibria

Equilibria — practice question

Gallic acid, $\text{C}_7\text{H}_6\text{O}_5$, is an aromatic compound found in nature. Its structure is displayed.
(a)[2]

Gallic acid has both carboxylic acid and phenol functional groups. State and explain which of these two groups is the stronger acid.

(b(i))[2]

A buffer was prepared by dissolving $2.04\,\text{g}$ of gallic acid in $250\,\text{cm}^3$ of a solution containing $0.0600\,\text{mol dm}^{-3}$ of gallate ions, $\text{C}_7\text{H}_5\text{O}_5^-$. The equilibrium is: $\text{C}_7\text{H}_6\text{O}_5 \rightleftharpoons \text{C}_7\text{H}_5\text{O}_5^- + \text{H}^+$ $K_a = 3.89 \times 10^{-5}\,\text{mol dm}^{-3}$ at $298\,\text{K}$. Define the term buffer solution.

(b(ii))[3]

Calculate the pH for this buffer solution.

(b(iii))[2]

Write two equations to show how a solution containing gallic acid, $\text{C}_7\text{H}_6\text{O}_5$, and gallate ions, $\text{C}_7\text{H}_5\text{O}_5^-$, functions as a buffer.

(c(i))[1]

Nitronium ions, $\text{NO}_2^+$, are formed by the reaction between concentrated sulfuric acid and concentrated nitric acid. Construct an equation for making $\text{NO}_2^+$ by this method.

(c(ii))[2]

Complete the mechanism and draw the intermediate for step 1. Include all relevant charges and curly arrows to show electron-pair movement.

(c(iii))[1]

State the mechanism name in (c)(ii).

(c(iv))[2]

State the reagents and the conditions used in step 4.

(c(v))[2]

Suggest a structure for compound $Q$ in the box provided.

(c(vi))[1]

State the reagents and the conditions for step 6.

(d(i))[1]

State how many peaks would be seen in the $^{13}\text{C}$ NMR spectrum of gallic acid.

(d(ii))[2]

The proton NMR spectrum of gallic acid dissolved in $\text{D}_2\text{O}$ is recorded. Predict how many peaks will be observed and any splitting pattern expected. State the expected chemical shift range $(\delta)$ for each predicted peak.

Worked solution & mark scheme

This 21-mark question has a full step-by-step worked solution and mark scheme. One marking point: Carboxylic acid is more acidic than phenol; explanation based on bond strength, inductive effect, or delocalisation

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