= Enantiomeric excess
{title2=$\mathrm{ee}$}
{wiki}
The <enantiomeric excess> of an <enantiomer> pair is the magnitude of the difference between their relative <mole fractions>, $\mathrm{ee}=|x_+-x_-|$, often reported as a percentage. Here $x_++x_-=1$ counts the pair only, excluding <solvent>; the signs label <optical rotation> under the specified conditions, not <R and S configuration>.
In the model above, $x_+=0.75$ and $x_-=0.25$ give $\mathrm{ee}=0.50=50\%$. Keep the same total pair <concentration> $c=0.200\,\mathrm{g\,mL^{-1}}$, path length, <solvent>, <temperature> and <wavelength>. The two contributions are
$$
c_+=0.150,\quad c_-=0.050\quad(\mathrm{g\,mL^{-1}}),\qquad
\alpha=40(1)(0.150-0.050)^\circ=+4^\circ.
$$
The $+8^\circ$ pure-member reference therefore gives $\alpha/\alpha_{\mathrm{pure},+}=+0.50$. This ratio measures signed population imbalance only for a pure pair with additive linear responses under matching conditions. Adding unrelated <solutes>, changing total <concentration>, or changing <chemical equilibrium> can invalidate that inference. For $x_+=x_-=0.50$, the <racemate> gives zero net <optical rotation> by cancellation; each <molecule> remains <chiral>. A zero reading alone proves neither a <racemate> nor molecular <achirality>.
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