= Solution
A $3d^9$ shell has one hole in the otherwise filled $d$ shell. <Hund's rules> give $S=1/2$, $L=2$, and, for a more-than-half-filled shell, $J=L+S=5/2$: the free-ion term is ${}^2D_{5/2}$. Its <Landé g-factor>[Landé factor] is
$$
g_J=1+\frac{J(J+1)+S(S+1)-L(L+1)}{2J(J+1)}=\frac65.
$$
Distinguish the <Effective paramagnetic moment>[Curie effective moment] from a saturated projection:
$$
\boxed{\mu_{\rm eff}^{\rm free}=g_J\mu_B\sqrt{J(J+1)}=3.55\mu_B,
\qquad\mu_{\rm sat}^{\rm free}=g_JJ\mu_B=3\mu_B.}
$$
The question then explicitly asks us to use <orbital quenching> in the <crystal>. <Crystal field theory>[Ligand fields] split and mix orbital states, suppressing their freely orientable <angular momentum>. The <Spin-only magnetic moment>[spin-only] approximation has $g\simeq2$, $S=1/2$, so $\mu_{\rm eff}=\sqrt3\mu_B$ but the two <Spin one-half along an axis>[field-axis] projections are $\pm\mu_B$. The latter, not $\sqrt3\mu_B$, belong in the tanh argument and <Spin-one-half paramagnetic magnetization>[saturation magnetization].
The sulfate and water environment supplies mainly closed-shell, weakly magnetic surroundings, stabilizes localized Cu2+ sites and separates their moments so that <Exchange interaction>[exchange] can be weak on the <temperature> scale considered. <Paramagnetism> here is an approximation of weakly interacting local spins, not a claim that dilution guarantees no magnetic ordering at any <temperature>. Elemental <copper> has a filled $3d$ band and itinerant <Conduction electron>[conduction electrons] rather than one localized Cu2+ spin per <Chemical atom>[atom]. <Pauli paramagnetism>[Pauli spin response] and orbital/core contributions then replace a local-spin <Curie's law>[Curie law]; ordinary pure <copper> has a weak net <Diamagnetism>[diamagnetic] response.https://www.sciencedirect.com/science/article/pii/S1090780715000774[2015 primary measurement, abstract: negative volume susceptibility of high-purity copper wire]{ref}
There is one Cu2+ per formula unit, hence $n_s=N_A/V_m=5.5148\times10^{27}$ m$^{-3}$. The <Spin-only magnetic moment>[spin-only] weak-field SI <Volume magnetic susceptibility>[volume susceptibility] is
$$
\chi_0(T)=\frac{\mu_0n_s\mu_B^2}{k_BT}=\frac{0.0431708\ \mathrm K}{T},
\qquad\boxed{\chi_0(300\ \mathrm K)=1.4390\times10^{-4},\quad
\chi_0(2\ \mathrm K)=2.1585\times10^{-2}.}
$$
These neglect <Exchange interaction>[exchange], <Anisotropy>[anisotropic] $g$ factors and background <diamagnetism>. At high field and low <temperature>, saturation requires the nonlinear response in <3/a>; a field-independent <Curie's law>[Curie value] then describes only the zero-field slope.
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