$a)\text{ PTHH: } 4P + 5_{O_{2}}\quad \underrightarrow{t^{o}} \quad 2P_{2}O_{5}$
$b) n_{P_{2}O_{5}}=\frac{42,6}{142}=0,3\text{ (mol)}$
$\text{Theo ptpư: }n_{P}=2n_{P_{2}O_{5}}=0,6\text{ (mol)}$
$\Rightarrow m_{P}=0,6.31=18,6\text{ (g)}$
$c) n_{O_{2}}=\frac{5}{2}n_{P_{2}O_{5}}=0,75\text{ (mol)}$
$\Rightarrow V_{O_{2}}=0,75.22,4=16,8\text{(l)}$