= Solution
For <fugacity coefficient> $\phi=0.75$, use $f=\phi P=3375\,\mathrm{bar}$ for the water activity. The solid pressure correction still uses the actual pressure $4500\,\mathrm{bar}$. Thus
$$
\boxed{T=\frac{130862.28}
{170.7-8.3145\ln[3375(0.20/0.80)^3]}
=950.131\,\mathrm K\simeq676.981\,{}^\circ\mathrm C.}
$$
Lowering the water <chemical potential> favours the product side, which lowers the equilibrium temperature relative to part (c) under the stated constant-property model.
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