$ 1/$
$4P+5O2\overset{t^o}{\longrightarrow}2P2O5$
$ 2/$
$n_P=12,4/31=0,4mol$
$PTHH:$
$4P+5O2\overset{t^o}{\longrightarrow}2P2O5$
$0,4$ $→ 0,2$
$⇒m_{P2O5}=0,2.142=28,4g$
$3/$
$PTHH:$
$4P+5O2\overset{t^o}{\longrightarrow}2P2O5$
$0,4 →0,2$
$⇒V_{O2}=0,2.32=4,48l$
$4/$
$n_{O2}=6,72/22,4=0,3mol$
$PTHH:$
$P+5O2\overset{t^o}{\longrightarrow}2P2O5$
$0,3 → 0,12$
$⇒mP2O5=0,12.142=17,04g$