= Solution
In the usual high-surface-potential <Schulze-Hardy rule> approximation, $c_{\rm ccc}\propto z^{-6}$, where $z$ is the magnitude of the counterion valence. Consequently,
$$
\boxed{c_{2:2}=\frac{100}{2^6}=1.56\,\mathrm{mM},\qquad c_{3:3}=\frac{100}{3^6}=0.137\,\mathrm{mM}.}
$$
Multivalent counterions screen surface charge much more effectively and strongly lower the electrostatic barrier. The sixth-power estimate includes the way the critical <ionic strength> and screening condition change with valence; merely scaling the ordinary <Debye length> at fixed salt concentration would not derive it. Specific adsorption, charge regulation or <ion> correlations can produce substantial deviations.<Schulze-Hardy rule in the IUPAC Gold Book>[Counterion-valence sensitivity of the <critical coagulation concentration>]{ref}
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