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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 ${\mathrm{C}}_8{\mathrm{H}}_6{\mathrm{O}}_4$. The positions of the groups are part of its identity, not information recoverable from the elemental formula alone.^[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.^[ref]

An original repeat-unit bookkeeping calculation isUsing rounded masses, $166.13+62.07-2(18.015)=192.17\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$ 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 $n(192.17)+18.015\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$ 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.