A student prepared a sample of soap in the school laboratory. The experiment was carried out in the four stages illustrated on the previous page. At Stage 1, the contents were refluxed for approximately 20 minutes 4.45 g of glycerol tristearate (an animal fat) together with an excess of sodium hydroxide pellets, anti-bumping material and about 30 cm² of ethanol. The reaction shown in the following balanced equation took place. $$C_{18}H_{36}O_6 + 3NaOH \rightarrow 3C_{17}H_{35}COONa + \text{soap (sodium stearate)} + \text{co-product}$$ The apparatus was then allowed to cool and rearranged for Stage 2, distillation, again using a water bath. After distillation, the contents of the distillation flask were deemed or washed into a beaker containing brine - Stage 3. Filtration was used in Stage 4 to isolate the soap which was then thoroughly washed. a) What is the purpose of refluxing in Stage 1 of the preparation? b) What substance was removed by distillation in Stage 2? c) Explain the role of the brine in Stage 3. d) Why was it necessary to wash the soap thoroughly in Stage 4? e) Draw the structure of glycerol. f) What was the co-product formed at the end of the reaction? g) Given that the mass of the sample obtained was 4.59 g, calculate the maximum yield in grams of soap that could have been obtained in this preparation. h) With reference to the structure of soap, explain why the hydrocarbon part is non-polar (hydrophobic) and dissolves oils (non-polar substances).

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A student prepared a sample of soap in the school laboratory - Leaving Cert Chemistry - Question 2 - 2014

Question 2

A student prepared a sample of soap in the school laboratory. The experiment was carried out in the four stages illustrated on the previous page. At Stage 1, the con... show full transcript

Worked Solution & Example Answer:A student prepared a sample of soap in the school laboratory - Leaving Cert Chemistry - Question 2 - 2014

Step 1

What is the purpose of refluxing in Stage 1 of the preparation?

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Answer

The purpose of refluxing is to allow ample time for the reaction to proceed to completion. It ensures maximum yield while preventing the loss of volatile materials such as ethanol. This process helps maintain a steady reaction temperature, which is crucial for achieving a thorough conversion of the reactants into products.

Step 2

What substance was removed by distillation in Stage 2?

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Answer

The substance removed by distillation in Stage 2 is ethanol. Ethanol is a volatile solvent that evaporates during distillation, allowing it to be separated from the other non-volatile substances present in the mixture.

Step 3

Explain the role of the brine in Stage 3.

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Answer

The role of the brine in Stage 3 is to precipitate or separate the soap from the aqueous mixture. The high concentration of salt in brine enhances the solubility difference between soap and water, thus aiding in the isolation of the soap which is insoluble in brine.

Step 4

Why was it necessary to wash the soap thoroughly in Stage 4?

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Answer

It was necessary to wash the soap thoroughly in Stage 4 to remove any residual sodium hydroxide (NaOH) which is corrosive and could cause skin irritation. Additionally, this washing process helps eliminate impurities and any left-over brine that may affect the quality of the final product.

Step 5

Draw the structure of glycerol.

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Answer

The structure of glycerol (propane-1,2,3-triol) is:

   H   OH   H
   |    |    |
H--C--C--C--H
   |    |    |
  OH   OH   H

In a more condensed form, it can be represented as C₃H₈O₃, reflecting its three hydroxyl (-OH) groups.

Step 6

What was the co-product formed at the end of the reaction?

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Answer

The co-product formed at the end of the reaction was glycerol (glycerin). It is generated during the saponification process when triglycerides (like glycerol tristearate) react with NaOH.

Step 7

Given that the mass of the sample obtained was 4.59 g, calculate the maximum yield in grams of soap that could have been obtained in this preparation.

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Answer

To calculate the maximum yield, we first determine the moles of glycerol tristearate used:

Molar mass of glycerol tristearate (C₁₈H₃₆O₆) =
Given mass = 4.45 g

Moles of glycerol tristearate = $\frac{4.45 g}{\text{Molar mass}}$

Using the stoichiometry from the balanced equation, determine the moles of soap produced. Finally, convert the moles of soap back to grams to find the maximum possible yield.

Step 8

With reference to the structure of soap, explain why the hydrocarbon part is non-polar (hydrophobic) and dissolves oils (non-polar substances).

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Answer

The hydrocarbon part of soap is non-polar because it consists of long chains of carbon and hydrogen atoms, which have very low electronegativity differences. This lack of polarity allows these hydrocarbon chains to dissolve non-polar substances such as oils, which also lack polar bonding. Thus, the soap can effectively mix with and remove these non-polar substances, making it effective as a cleansing agent.

A student prepared a sample of soap in the school laboratory - Leaving Cert Chemistry - Question 2 - 2014

Worked Solution & Example Answer:A student prepared a sample of soap in the school laboratory - Leaving Cert Chemistry - Question 2 - 2014

What is the purpose of refluxing in Stage 1 of the preparation?

What substance was removed by distillation in Stage 2?

Explain the role of the brine in Stage 3.

Why was it necessary to wash the soap thoroughly in Stage 4?

Draw the structure of glycerol.

What was the co-product formed at the end of the reaction?

Given that the mass of the sample obtained was 4.59 g, calculate the maximum yield in grams of soap that could have been obtained in this preparation.

With reference to the structure of soap, explain why the hydrocarbon part is non-polar (hydrophobic) and dissolves oils (non-polar substances).

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