$a) 2C_{2}H_{2} + 5O_{2} \underrightarrow{t^{o}} 4CO_{2} + 2H_{2}O$
$b) n_{C_{2}H_{2}}=\frac{20}{22,4}.97\%=\frac{97}{112} (mol)$
$n_{O_{2}}=\frac{5}{2}n_{C_{2}H_{2}}=\frac{485}{224} (mol)$
$\Rightarrow V_{O_{2}}=\frac{485}{224}.22,4=48,5 (l)$
$c) n_{CO_{2}}=2n_{C_{2}H_{2}}=\frac{97}{56} (mol)$
$n_{H_{2}O}=n_{C_{2}H_{2}}=\frac{97}{112} (mol)$
$\Rightarrow m_{CO_{2}}=\frac{97}{56}.44=\frac{1067}{14} (g)$
$\Rightarrow m_{H_{2}O}=\frac{97}{112}.18=\frac{873}{56}$