= Terephthalic acid
{title2=$\mathrm{C_8H_6O_4}$}
\b[<Terephthalic acid>] is benzene-1,4-dicarboxylic acid, an aromatic <dicarboxylic acid> with two <carboxyl groups> attached at opposite positions of a benzene ring. Its neutral <molecular formula> is $\mathrm{C_8H_6O_4}$. The positions of the groups are part of its identity, not information recoverable from the elemental formula alone.<Terephthalic acid in ChEBI>[CHEBI:15702, definition, structure and formula]{ref}
The two <carboxyl groups> make it a useful difunctional building block. Reaction with <ethylene glycol>, which has two alcohol groups, forms <ester> connections in <polyethylene terephthalate>, a <polyester>. This is <step-growth polymerization>: growing molecules react through their remaining functional groups.<Organic Chemistry by John McMurry>[Section31.4, Step-Growth Polymers, terephthalic acid and ethylene glycol example]{ref}
An original repeat-unit bookkeeping calculation is
$$
\mathrm{C_8H_6O_4+C_2H_6O_2-2H_2O=C_{10}H_8O_4}.
$$
Using rounded masses, $166.13+62.07-2(18.015)=192.17\,\mathrm{g\,mol^{-1}}$ per repeat-unit contribution. The subtraction represents two <esterification> connections per repeat in the long-chain description; it is not an elementary reaction equation for one isolated pair of monomers forming a complete polymer.
For a finite linear chain assembled from $n$ acid molecules and $n$ glycol molecules, with no ring closure or side reaction, there are $2n-1$ connections. The chain therefore loses $2n-1$ <water> molecules, rather than $2n$. End groups supply the difference: its mass is approximately $n(192.17)+18.015\,\mathrm{g\,mol^{-1}}$ under the same rounded-mass convention.
The <molar mass> and end-group balance consequently matter alongside conversion. Measuring consumption of acid groups and formation of ester connections tests the reaction model; identifying an aromatic diacid does not determine the final chain length or material properties.
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