Coconut oil contains a triester with three identical R groups - AQA - A-Level Chemistry - Question 1 - 2021 - Paper 2

The compound RCOOK is a potassium carboxylate salt.

One use for potassium carboxylate salts is in the production of soap, where they act as surfactants.

To determine the value of n:

1. The molar mass of R, from the triester, can be calculated as follows:
   • M = 638 g/mol
   • R has a structure of CH₃(CH₂)n, where the mass contribution is: $ext{C} = 12 ext{ g/mol, H} = 1 ext{ g/mol}$
   • The approximate mass of the alkyl group R: $12 + 2n + 1 = 12 + 2n + 1$
   • Therefore, we get: $638 = 3(12 + 2n + 1) + 58 ext{ (weight of K and O atoms)}$
   • Solving gives n = 14.

To find the percentage by mass:

1. Calculate the amount of KOH reacted:
   • Moles of KOH = mass / molar mass = 0.421 g / 56.1 g/mol = 0.00749 mol
2. Calculate the moles of HCl:
   • Moles of HCl = 15.65 cm³ × 0.100 mol/dm³ = 0.001565 mol
3. Therefore, moles of KOH reacted is:
   • 0.00749 - 0.001565 = 0.005925 mol
4. Molar mass of the triester = 638 g/mol
5. Mass of triester in sample:
   • Mass = moles × molar mass = 0.005925 mol × 638 g/mol = 3.78 g
6. Percentage by mass:
   • Percentage = (mass of triester / mass of coconut oil) × 100
   • Percentage = (3.78 g / 1.450 g) × 100 = 261.38%, which indicates the presence of more than one triester in the sample.

Aqueous ethanol is suitable because it allows the KOH to dissolve effectively and facilitates the hydrolysis of the triester.

One safety precaution when heating the mixture is to use a fume hood or ventilation to avoid inhalation of vapors.

Using a fume hood prevents the inhalation of harmful fumes or vapors produced during the heating process, thereby ensuring the safety of the experimenter.