= Malonic acid
{title2=$\mathrm{C_3H_4O_4}$}
\b[<Malonic acid>] is propanedioic acid, a <dicarboxylic acid> in which two <carboxyl groups> are separated by one methylene group:
$$
\mathrm{HOOC{-}CH_2{-}COOH}.
$$
Its neutral <molecular formula> is $\mathrm{C_3H_4O_4}$ and its average <molar mass> is approximately $104.061\,\mathrm{g\,mol^{-1}}$.<Malonic acid in ChEBI>[CHEBI:30794, definition, formula and structure]{ref}
Two of the four hydrogen atoms belong to carboxyl OH groups; the other two belong to the central carbon. Complete neutralization of both carboxyl groups therefore requires two equivalents of a suitable base, not four merely because the formula contains four hydrogens. An original stoichiometric model is
$$
\mathrm{H_2A+2OH^-\longrightarrow A^{2-}+2H_2O}.
$$
For $1.00\,\mathrm{mmol}$ acid, the two-equivalent amount of $0.100\,\mathrm{mol\,L^{-1}}$ hydroxide solution is $20.0\,\mathrm{mL}$. The <pH> at an experimental endpoint also depends on acid-base equilibria and the measurement method; the equivalent count alone does not determine it. <Malonate> names the corresponding deprotonated system, whose protonation state should be specified.
Heating can promote <decarboxylation>, producing <acetic acid> and <carbon dioxide>. Malonic-acid derivatives underlie the carbon-chain construction described in the malonic ester synthesis.<Organic Chemistry by John McMurry>[Section22.7, Alkylation of Enolate Ions, malonic ester hydrolysis and decarboxylation]{ref} For the unsubstituted acid, the stoichiometric balance is
$$
\mathrm{C_3H_4O_4\longrightarrow C_2H_4O_2+CO_2}.
$$
An original ideal mass balance for $1.00\,\mathrm{mmol}$ converted by only this reaction gives about $60.052\,\mathrm{mg}$ <acetic acid> and $44.009\,\mathrm{mg}$ <carbon dioxide> from $104.061\,\mathrm{mg}$ starting material. Following evolved gas and residual material would test conversion, whereas a change in sample mass alone would not distinguish this reaction from evaporation or another decomposition pathway.
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