дано
m практ (SO3) = 20 g
η(SO3) = 80%
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V(SO2)-?
m теор (SO3) = 20*100% / 80% = 25 L
2SO2+O2-->2SO3
M(SO3) = 80 g/mol
n(SO3) = m/M = 25/ 80 = 0.31 mol
2n(SO2) = 2n(SO3) = 0.31 mol
V(SO2) = n*Vm = 0.31 * 22.4 = 6.944 L
ответ 6.944 л