= Solution
For an ideal dilute <electrolyte> with molar <ionic strength> $I=\tfrac12\sum_i c_i z_i^2$, measured in $\mathrm{mol\,L^{-1}}$, the <Debye–Hückel screening length> is
$$
\kappa^{-1}=\left(\frac{\varepsilon_r\varepsilon_0RT}{2F^2(1000)I}\right)^{1/2}.
$$
Taking water's relative <permittivity> as approximately $78.5$ at $25^\circ\mathrm C$, $I=0.10\ \mathrm M$ for NaCl. Therefore \b[$\kappa^{-1}\simeq0.96\ \mathrm{nm}$]. This is an exponential screening <length>, not a hard outer edge of the diffuse layer. The factor of $1000$ converts molarity into $\mathrm{mol\,m^{-3}}$.
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