= Glycolic acid
\b[Glycolic acid], also called hydroxyacetic acid, is the <carboxylic acid> $\mathrm{HOCH_2COOH}$. It has a <hydroxy group> on the carbon immediately adjacent to its carboxyl carbon, making it an <alpha-hydroxy acid>. NIST records its neutral <molecular formula> as $\mathrm{C_2H_4O_3}$ and CAS number as 79-14-1.<Glycolic acid in the NIST Chemistry WebBook>[Identity fields, structural InChI and synonyms]{ref}
The two oxygen-containing groups have different connectivity. One is the alcohol-like $\mathrm{HO{-}CH_2{-}}$ group; the other is the $\mathrm{{-}C(=O)OH}$ acid group. <Ethylene glycol>, $\mathrm{HOCH_2CH_2OH}$, instead has two alcohol groups and formula $\mathrm{C_2H_6O_2}$. The related names do not make them the same compound.
Formal loss of the carboxyl proton gives <glycolate>:
$$
\mathrm{HOCH_2COOH}\rightleftharpoons\mathrm{H^+}+\mathrm{HOCH_2COO^-}.
$$
This changes charge and hydrogen count. A glycolate salt also includes a charge-balancing <counterion>; its formula and <molar mass> should not be substituted for those of the neutral acid.
Using the recorded neutral-acid molar mass about $76.0514\,\mathrm{g\,mol^{-1}}$, an original $1.00\,\mathrm{mmol}$ amount corresponds to about $76.05\,\mathrm{mg}$. Atom bookkeeping also clarifies its relation to <polyglycolic acid>. Forming one ester link between two glycolic-acid units with formal loss of one <water> molecule gives composition
$$
2\,\mathrm{C_2H_4O_3}-\mathrm{H_2O}=\mathrm{C_4H_6O_5}.
$$
For a linear chain made from $n$ such units with $n-1$ formal water losses, the composition is $\mathrm{C_{2n}H_{2n+2}O_{2n+1}}$. This expresses ideal chain/end-group bookkeeping, not an industrial synthesis procedure.
The manufacturer's <polyglycolic acid> brochure gives the polymer repeat structure and identifies glycolic acid as its hydrolysis product.<Kuredux Polyglycolic Acid Resin technical brochure by Kureha>[PDF p. 2, repeat structure and hydrolytic decomposition]{ref} A polymer repeat unit and the isolated acid molecule differ by the atoms associated with linkage formation and end groups.
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