Problem 46 - General Chemistry: Principles and Modern Applications

Question

Explain why Dacron is called a polyester. What is the percent oxygen, by mass, in Dacron?

Step-by-Step Solution

Answer

Dacron is referred to as a polyester because of its molecular structure consisting of repeated esters – carbon, hydrogen and oxygen atoms. The percent oxygen by mass in Dacron, also known as Polyethylene Terephthalate, can be calculated using the formula of Dacron and the atomic masses of Carbon, Hydrogen and Oxygen.

Step 1: Understanding Polyesters

Polyesters are polymers formed from the reaction between dicarboxylic acids and diols. The structural formula of these polyesters often involve repeated ester (C=O and O) groups. Dacron, specifically, is a type of polyester famously known as Polyethylene Terephthalate (PET). The PET is formed by the condensation of terephthalic acid and ethylene glycol.

Step 2: Calculating the Molar Mass of Dacron

Determine the molar mass of PET. Using the periodic table, find the atomic masses of the atoms that make up PET: Carbon (C), Hydrogen (H), and Oxygen (O). For PET, the formula is \(C_{10}H_{8}O_4\), so the molar mass of PET is calculated as follows: \( mM(PET) = 10*mM(C) + 8*mM(H) + 4*mM(O) \)

Step 3: Calculating the Molar Mass of Oxygen in Dacron

Next, calculate the molar mass contributed by the oxygen atoms alone in the Dacron: \( mM(O in PET) = 4*mM(O) \)

Step 4: Calculating the Oxygen percentage by Mass in Dacron

Finally, to determine the percent oxygen, by mass, in PET, divide the molar mass of Oxygen by that of PET and multiply the result by 100: \( Percent(O) = \frac{mM(O in PET)}{mM(PET)} * 100 \)

Key Concepts

Polyethylene Terephthalate (PET)

Polyethylene Terephthalate, commonly known as PET, is a type of polyester. Polyesters are an essential class of polymers formed through a chemical reaction between dicarboxylic acids and diols, where esters are the characteristic units. One of the most well-known forms of polyester is PET, celebrated for its use in everyday items like drink bottles and clothing fibers. This compound is specifically crafted through a condensation reaction between terephthalic acid and ethylene glycol.

• Terephthalic acid provides the structural rigidness due to its aromatic properties.
• Ethylene glycol is responsible for flexibility and durability of the polymer chains.

PET showcases excellent thermal stability and barrier properties, making it highly desirable for packaging applications. Understanding PET involves knowing its robust structure marked by repeating ester (C=O and O) linkages.

Molar Mass Calculation

Calculating the molar mass of a compound is fundamental to understanding its chemical properties. The molar mass is a measure of the mass of one mole of substance, expressed in grams per mole. To calculate the molar mass of Polyethylene Terephthalate (PET), we analyze its chemical formula, which is \[ C_{10}H_{8}O_4 \].Each element in the formula contributes to the total mass:

• Carbon (C): 10 atoms, with an atomic mass of approximately 12.01 g/mol.
• Hydrogen (H): 8 atoms, with an atomic mass of approximately 1.01 g/mol.
• Oxygen (O): 4 atoms, with an atomic mass of approximately 16.00 g/mol.

Using these atomic masses, we calculate the total molar mass:\[ mM(PET) = 10 \times 12.01 + 8 \times 1.01 + 4 \times 16.00 \]Understanding molar mass allows scientists and engineers to figure out how much of each element is present in a given amount of PET.

Oxygen Percentage by Mass

The percentage of oxygen by mass in a compound like PET provides insights into its composition and how it may react chemically. Knowing how much oxygen is in PET reveals important properties like its combustibility or how it may engage in chemical reactions. To compute the oxygen percentage:

• First, calculate the molar mass of the oxygen in PET:
  \[ mM(O \text{ in PET}) = 4 \times 16.00 \]
• Then, divide the molar mass of the oxygen by the overall molar mass of PET:
  \[ Percent(O) = \frac{mM(O \text{ in PET})}{mM(PET)} \times 100 \]

This percentage tells us how much of the polymer's weight is due to oxygen atoms, which in even small amounts can influence the material's characteristics dramatically. For example, in plastics, a higher oxygen content can increase the polymer's susceptibility to degradation when exposed to the environment.