Question

Why would a repeating unit of PET (polyethyleneterephthalate) be used to calculate the limiting reagent and yield instead of the molecular weight of the polymer? Is it just because it is a polymer and only its monomer needs to be taken into account?

Answer 1

If you treat the repeating units in the polymer as if they were individual molecules, the analysis comes out the same.

To find the limiting reagent , you just calculate how many moles of the reactants you have and in what ratio they combine to give one RU(repeating unit ) of the product polymer. And you are there, ..

For example, in PET is polymerized unit of monomers, ethylene glycol and terepthalic acid in 1:1 ratio,

HO-CH2-CH2-OH +HOOC-C6H4-COOH= [-CH2-CH2-OOC-C6H4-COO-]n (ethylene terepthalate.

If you know that you begin with 5 moles of ethylene glycol ,and 10 moles of terepthalic acid, you would still get 5 moles of product as they combine in 1:1 ratio.

So ethylene glycol is the limiting reagent.

While calculating theoretical yield also, you need to convert the moles of product (5 moles here) in grams by using the molecular weight of the RU in product, which gives an estimate of the yield of the molecule as a whole.