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 Incoming links: Hydrate

Hydration state 2026-10-31

The number of water units included in a specified crystalline hydrate formula. A weighed hydrate and its anhydrous compound have different molar masses.

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Past exam of the medicine course of the University of Oxford / 1864 / First examination / Chemistry / 4 / Solution 2026-10-31

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In modern notation the formulas are HCN, HCOOH, $H_{2} C_{2} O_{4}$ , $H g_{2} C l_{2}$ , $HgC l_{2}$ , $A s_{2} O_{3}$ , a hydrated potassium antimony tartrate, and $KCl O_{3}$ , respectively. Several historical names need qualification.

Hydrocyanic acid is hydrogen cyanide, $HCN$ . The pharmaceutical name normally concerns an aqueous chemical solution; its concentration is not encoded by the molecular chemical formula.
Formic acid is $HCOOH$ , equivalently $C H_{2} O_{2}$ .
Anhydrous oxalic acid is $H_{2} C_{2} O_{4}$ , or $(COOH)_{2}$ . Its common crystalline dihydrate is $H_{2} C_{2} O_{4} \cdot 2 H_{2} O$ ; the water of crystallization must be included when calculating a weighed quantity.
Calomel is mercury(I) chloride, $H g_{2} C l_{2}$ . The formula $HgCl$ gives only the simplest composition ratio and conceals the paired mercury(I) ion.
Corrosive sublimate is mercury(II) chloride, $HgC l_{2}$ . It is chemically distinct from calomel.
The solid historically sold as arsenious acid is normally arsenic trioxide, $A s_{2} O_{3}$ , often written $As O_{3}$ in older equivalent notation. The genuinely hydrated molecular arsenous acid is $H_{3} As O_{3}$ , also written $As (OH)_{3}$ . One should not quietly assign the hydrated formula to the dry commercial oxide.
Tartar emetic, or potassium antimony tartrate, is conventionally represented as $K (SbO) C_{4} H_{4} O_{6} \cdot \frac{1}{2} H_{2} O$ . A structurally more informative doubled formula is $K_{2} S b_{2} (C_{4} H_{2} O_{6})_{2} \cdot 3 H_{2} O$ : the complete elemental totals agree. The older “antimonyl tartrate” formula is a compositional convention and does not imply isolated $Sb O^{+}$ units in the crystal.^[ref]
Chlorate of potash is potassium chlorate, $KCl O_{3}$ .

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

HO

. Such formulas must be read together with their contemporary atomic weights, rather than treated as inconsistent modern molecular formulas.

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