A Fe$-$P liquid alloy is in equilibrium with a solid solution and a gas phase at $1580C.$ The solid solution

behaves ideally and contains two components, CaO and Ca$3$P$2$O$8(3$CaO.P$2$O$5).$ The P content in the

solid solution is $2\mathrm{.}4$ wt$.\%.$ The gas phase includes $4\mathrm{.}810-10$ atm oxygen. The activity of $P$ in the alloy,

with respect to the $1$ wt$.\%$ in Fe standard state in $Fe$ is $28\mathrm{.}04.$

The given data follows: