= Solution
{c}
Write the fitted inverse <molar magnetic susceptibility> as
$$
\chi_{\rm mol}^{-1}=sT+b=\frac{T-\Theta}{C_{\rm mol}},\qquad
C_{\rm mol}=\frac1s,\qquad \Theta=-\frac bs.
$$
The vertical scale is in $10^6\,\mathrm{mol\,m^{-3}}$, not unscaled inverse SI <magnetic susceptibility>. The source line is approximately $8.8\times10^6\,\mathrm{mol\,m^{-3}}$ at $100\,\mathrm K$ and $19\times10^6\,\mathrm{mol\,m^{-3}}$ at $300\,\mathrm K$. These readings give
$$
s\simeq5.1\times10^4\,\mathrm{mol\,m^{-3}\,K^{-1}},\qquad
b\simeq3.7\times10^6\,\mathrm{mol\,m^{-3}}.
$$
Therefore the <effective paramagnetic moment> and <Weiss temperature> are
$$
\boxed{m_{\rm eff}=\sqrt{\frac1{(1.57\times10^{-6})s}}
\simeq3.5\text{–}3.6,\qquad
\Theta\simeq-70\,\mathrm K.}
$$
Equivalently, $\mu_{\rm eff}\simeq(3.5\text{–}3.6)\mu_B$. An uncertainty of order $10\,\mathrm K$ in the graphical <Weiss temperature> is reasonable; the plotted fit does not justify exact integer parameters. Its positive intercept at $T=0$ requires a negative extrapolated <temperature>-axis intercept. Thus the dominant interactions have the <antiferromagnetic> sign in the interpretation of part (b).
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