= Stilbene
\b[Stilbene] is 1,2-diphenylethene: two <phenyl groups> attached to the two carbons of an ethene <double bond>, with connectivity $\mathrm{C_6H_5{-}CH{=}CH{-}C_6H_5}$. Its <molecular formula> is $\mathrm{C_{14}H_{12}}$. The connectivity admits two <geometric isomers>, <cis-Stilbene> and <trans-Stilbene>, which the NIST identity records distinguish despite their equal formulas and <molar masses>.<cis-Stilbene in the NIST Chemistry WebBook>[Identity, Z-isomer InChI and stereoisomer cross-reference]{ref}<trans-Stilbene in the NIST Chemistry WebBook>[Identity, E-isomer InChI and stereoisomer cross-reference]{ref}
Each central carbon bears one <phenyl group> and one <hydrogen> atom. In <cis-Stilbene>, the two <phenyl groups> lie on the same side of the <double bond>; in <trans-Stilbene>, they lie on opposite sides. The <Cahn-Ingold-Prelog priority rules> assign these arrangements Z and E respectively. Specifying only stilbene connectivity does not choose between them.
Atom counting provides a simple structural check:
$$
2\,\mathrm{C_6H_5}+\mathrm{C_2H_2}=\mathrm{C_{14}H_{12}}.
$$
Using $M\simeq180.245\,\mathrm{g\,mol^{-1}}$, an original $18.0245\,\mathrm{mg}$ sample corresponds to $0.1000\,\mathrm{mmol}$ of pure stilbene. That mass conversion is unchanged if the sample contains both <geometric isomers>; it cannot establish their proportions.
A <stilbene isomer composition measurement> needs information sensitive to geometry. For an ideal two-component optical example, let $c_E,c_Z$ be the two concentrations and let $a_j$ be background-subtracted <absorbance> divided by path length at wavelength $j$. A calibrated <Beer-Lambert law> model gives
$$
\begin{pmatrix}a_1\\a_2\end{pmatrix}
=\begin{pmatrix}\epsilon_{E1}&\epsilon_{Z1}\\\epsilon_{E2}&\epsilon_{Z2}\end{pmatrix}
\begin{pmatrix}c_E\\c_Z\end{pmatrix}.
$$
The two concentrations can be separated by this model only if $\epsilon_{E1}\epsilon_{Z2}-\epsilon_{E2}\epsilon_{Z1}\ne0$. Proportional response columns would leave composition unidentifiable even with perfectly repeated readings. The coefficients must be established for the solvent and measurement conditions; the example does not assign experimental spectra.
<Resveratrol> illustrates how the backbone recurs in related chemistry: replacing three aromatic hydrogens by <hydroxy groups> adds three oxygens while retaining a central <double bond> capable of <cis-trans isomerism>. Molecular composition, connectivity and geometry are distinct layers of chemical identification.
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