IMAT 2021 Q25 [Fatty Acids]

Three fatty acids with the formula C8H16O2 react with glycerol (C3H8O3) to form a triglyceride.

Which of the following is the correct formula for the triglyceride ?

A. C27H50O6
B. C27H48O5
C. C27H48O6
D. C27H52O7
E. C27H56O9

The correct answer is A : C27H50O6

To answer this question, you have proceeded in a step-by-step manner. First, we have to know that glycerol + 3 fatty acids produce a triglyceride and three atoms of water (H2O).

There are (8 x 3) + 3 = 27 atoms of carbon in this reaction

There are (3 x 16) + 8 = 56 atoms of hydrogen in this reaction

  • But 3 x 2 = 6 of these atoms of hydrogen form the 3 molecules of H2O released by the reaction
  • So there are 56 - 6 = 50 atoms of hydrogen in the triglyceride

There are (3 x 2) + 3 = 9 atoms of oxygen in this reaction

  • But 3 of these atoms of oxygen form the 3 molecules of H2O released by the reaction
  • So there are 9 - 3 = 6 atoms of oxygen in the triglyceride

Therefore, the formula of the triglyceride is C27H50O6.

When an ester bond is formed, one molecule of water is produced for each bond. Since three fatty acids are reacting with the glycerol molecule, there will be three ester bonds formed and three water molecules produced.

The loss of these water molecules must be accounted for in the final formula of the triglyceride. Since each water molecule contains one oxygen atom and two hydrogen atoms, losing three water molecules will result in a loss of 3 oxygen atoms and six hydrogen atoms from the triglyceride formula.