.2 g glycerol tristearate and .18 g NaOH was used Pre-Lah Questions 1. Calculate

Question

.2 g glycerol tristearate and .18 g NaOH was used


Pre-Lah Questions 1. Calculate how much excess NaOH is used in the experiment. (Remember that one mol OH is needed per ester group saponified) 2. Predict what you expect to observe when a few crystals of CaCl2 are added to the soap solution 3. Predict what you expect to observe when a few crystals of CaCl, are added to a detergent solution Post-Lah Questions .Soap dissolved in water produces an alkaline solution: why? 2. Cheap soaps sometimes leave skin feeling/looking dry, and in some cases, raw and blistered. What might cause this effect?

Explanation / Answer

1. Glycerol tristearate is a triester of three equivalents of stearic acid with one equivalent glycerol owing to the three hydroxyl groups of the latter. This triester of fatty acid stearic acid thus requires one mole of hydroxide per ester bond hydrolysis giving a total of three equivalents of sodium hydroxide per equivalent of glycerol tristearate to completely hydrolyse the molecule.

The molar mass of glycerol tristearate is 891.501g/mol giving 2/891.501 = 2.2434mmoles of the triester in 2g of the compound taken. This implies that 3*2.2434mmoles = 6.7302mmoles of NaOH is the minimal required amount of base for the ester taken. This amounts to 6.7302*40 = 0.2692g of NaOH. Therefore, the excess NaOH for this reaction is 18-0.2692 = 17.7308g.

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