The method and the results are on the electrophilic addition lab. Do the analysis yourself first, then check the answers below.
Quick answer: Alkenes are hydrocarbons with a C=C double bond whose exposed π electrons attract electrophiles. These answers go with the Chemistry 11-12 lab on electrophilic addition: the graph, the calculations, the uncertainties and the conclusion explained, for all 7 analysis questions.
Also, the model answers use the sample table. In addition, keep three significant figures, show the blank correction and remember that 0.100 g of oil is scaled by a factor of 1 000 to reach 100 g.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: A Level 3.3.4
Corrected titre 24.1 − 0.3 = 23.8 cm³. n = 0.020 × 23.8 / 1000 = 4.76 × 10⁻⁴ mol.
Question 3 Per 100 g of oil
Scale by 100 / 0.100 = 1 000: 4.76 × 10⁻⁴ × 1 000 = 0.476 mol per 100 g. Mass = 0.476 × 159.8 = 76.1 g per 100 g.
Question 4 Coconut oil value
n = 0.020 × 1.6 / 1000 = 3.2 × 10⁻⁵ mol; × 1 000 = 0.032 mol per 100 g.
Question 5 Ranking and meaning
Order: sunflower, then rapeseed, olive, palm and coconut. Highly unsaturated oils are liquid and go rancid faster; coconut oil is nearly saturated, solid in cool weather and more stable when heated.
Question 6 Number of C=C per molecule
Triglyceride: 100 / 880 = 0.1136 mol per 100 g. C=C per molecule = 0.298 / 0.1136 = 2.6.
Question 7 Errors and improvements
Bromine is volatile and also reacts slowly with propanone, and the end point is judged by eye. Improve by using a colorimeter, running a blank (as here), covering the flasks and repeating until titres agree to 0.2 cm³.
