$11\mathrm{.}5$ When $S{O}_3$ is decomposed at the constant pressure $P$ and $T=1000K,$ the partial pressure of $O_2$ in the equilibrium gas is $0\mathrm{.}05$atm. What is the pressure $P?$ If the pressure of this equilibrated gas is increased to $1$atm, to what value must the temperature be decreased to produce a gas mixture in which $p{O}_2=0\mathrm{.}05$atm?