One molecule of phenylmethanol (benzyl alcohol) can be oxidised to one molecule of benzoic acid by potassium manganate(VII) in basic conditions as shown in the reaction scheme below - Leaving Cert Chemistry - Question 2 - 2015

The colour change observed is from purple (violet) to dark brown (muddy black) as the potassium manganate(VII) is reduced during the oxidation of phenylmethanol.

A is hydrochloric acid (HCl) and B is sodium sulfite (Na2SO3).

Upon the addition of hydrochloric acid (A), the dark brown colour disappears, indicating the reduction of manganese. When sodium sulfite (B) is added, white crystals (precipitate) form, which gradually appears and becomes colourless.

The oxidation number of manganese decreases from +7 in MnO4⁻ to +4 in MnO2 during the reaction.

To maximise the yield of recrystallisation, one should use the minimum amount of hot solvent needed to dissolve the benzoic acid completely before cooling.

Recrystallisation would typically result in a sharper melting point range, narrowed closer to the pure melting point of benzoic acid, around 122 °C.

First, calculate the moles of phenylmethanol:

Volume = 2.7 cm³ = 0.0027 L, Density = 1.04 g/cm³, Mass = Volume × Density = 2.7 × 1.04 = 2.808 g.

Molar mass of phenylmethanol (C7H8O) = 108.13 g/mol.

Moles of phenylmethanol = Mass / Molar mass = 2.808 g / 108.13 g/mol = 0.02593 moles.

From the reaction, 1 mole of phenylmethanol produces 1 mole of benzoic acid. Therefore, moles of benzoic acid produced = 0.02593 moles.

Mass of benzoic acid obtained = 1.83 g, Molar mass of benzoic acid (C7H6O2) = 122.12 g/mol.

Theoretical mass of benzoic acid = 0.02593 moles × 122.12 g/mol = 3.17 g.

Percentage yield = (Actual yield / Theoretical yield) × 100 = (1.83 g / 3.17 g) × 100 = 57.58%.