\(\begin{array}{l}
a)\\
2HgO\xrightarrow{t^o}2Hg+O_2\\
Thuy\,ngan\,oxit\dashrightarrow thuy\,ngan+oxi\\
b)\\
Phan\,ung\,phan\,huy\\
c)\\
n_{HgO}=\frac{21,7}{217}=0,1(mol)\\
n_{Hg}=n_{HgO}=0,1(mol)\\
n_{O_2}=\frac{1}{2}n_{Hg}=0,05(mol)\\
V_{O_2}=0,05.22,4=1,12(l)\\
m_{Hg}=0,1.201=20,1(g)\\
d)\\
4P+5O_2\xrightarrow{t^o}2P_2O_5\\
n_P=\frac{4}{5}n_{O_2}=0,04(mol)\\
m_P=0,04.31=1,24(g)
\end{array}\)