= Solution
\b[In modern notation the formulas are HCN, HCOOH, $\mathrm{H_2C_2O_4}$, $\mathrm{Hg_2Cl_2}$, $\mathrm{HgCl_2}$, $\mathrm{As_2O_3}$, a hydrated potassium antimony tartrate, and $\mathrm{KClO_3}$, respectively.] Several historical names need qualification.
* <Hydrocyanic acid> is <hydrogen cyanide>, $\boxed{\mathrm{HCN}}$. The pharmaceutical name normally concerns an aqueous <chemical solution>; its <concentration> is not encoded by the molecular <chemical formula>.
* <Formic acid> is $\boxed{\mathrm{HCOOH}}$, equivalently $\mathrm{CH_2O_2}$.
* Anhydrous <oxalic acid> is $\boxed{\mathrm{H_2C_2O_4}}$, or $\mathrm{(COOH)_2}$. Its common crystalline dihydrate is $\mathrm{H_2C_2O_4\cdot2H_2O}$; the <water of crystallization> must be included when calculating a weighed quantity.
* <Calomel> is <mercury(I) chloride>, $\boxed{\mathrm{Hg_2Cl_2}}$. The formula $\mathrm{HgCl}$ gives only the simplest composition ratio and conceals the paired <mercury(I) ion>.
* <Corrosive sublimate> is <mercury(II) chloride>, $\boxed{\mathrm{HgCl_2}}$. It is chemically distinct from <calomel>.
* The solid historically sold as <arsenious acid> is normally <arsenic trioxide>, $\boxed{\mathrm{As_2O_3}}$, often written $\mathrm{AsO_3}$ in older equivalent notation. The genuinely hydrated molecular <arsenous acid> is $\mathrm{H_3AsO_3}$, also written $\mathrm{As(OH)_3}$. One should not quietly assign the hydrated formula to the dry commercial <oxide>.
* <Tartar emetic>, or <potassium antimony tartrate>, is conventionally represented as $\boxed{\mathrm{K(SbO)C_4H_4O_6\cdot\tfrac12H_2O}}$. A structurally more informative doubled formula is $\mathrm{K_2Sb_2(C_4H_2O_6)_2\cdot3H_2O}$: the complete elemental totals agree. The older “antimonyl tartrate” formula is a compositional convention and does not imply isolated $\mathrm{SbO^+}$ units in the crystal.<Antimony Potassium Tartrate in PubChem>[compound 73415808, elemental formula C8H10K2O15Sb2]{ref}
* <Chlorate of potash> is <potassium chlorate>, $\boxed{\mathrm{KClO_3}}$.
The <chemical formulas> above use modern <atomic weights> and distinguish <molecular formulas>, <formula units> and <hydrates>. Historical equivalent notation sometimes halves the subscripts attached to divalent <chemical elements> or writes <water> as $\mathrm{HO}$. Such formulas must be read together with their contemporary <atomic weights>, rather than treated as inconsistent modern <molecular formulas>.
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