$n_{P_2O_5}=2,84/142=0,02mol$
$4P+5O_2\overset{t^o}\to 2P_2O_5$
$a/$
Theo pt :
$n_P=2.n_{P_2O_5}=2.0,02=0,04mol$
$⇒m_P=0,04.31=1,24g$
$b/$
Theo pt :
$n_{O_2}=5/2.n_{P_2O_5}=5/2.0,02=0,05mol$
$⇒V_{O_2}=0,05.22,4=1,12l$
$⇒V_{kk}=1,12.5=5,6l$
$c/$
$2KClO_3\overset{t^o}\to 2KCl+3O_2$
Theo pt :
$n_{KClO_3}=2/3.n_{O_2}=2/3.0,05=\dfrac{1}{30}mol$
$⇒m_{KClO_3}=\dfrac{1}{30}.122,5≈4,083g$